I've tried several times playing solo with my monk but always find it nigh impossible. I find myself effective as a supporting character but not as a main character for a defense setup. It tends to be extremely tricky because the monk has no wall summons. I have trouble coming up with ways to hold enemies back where I am not temporarily. If I throw up an ensnare and electric aura, electric immune mobs spawn and I can't deal with them. Enrage aura is finicky and never works as I would expect. Are there any effective strategies to actually play solo with a monk?
My brother and I are currently playing monks together, and while there are two of us a lot of this information might be useful. I will use the Deeper Well map as reference.
Alrighty, information about how awesome Enrage aura's are. These should be the first auras that creeps run into. If possible put them at choke points where multiple columns of creeps come together. For the Deeper Well this is at the bottom of the initial staircases. Combine them with an ensare if you can to give the ragers a good chance to attack. These also act as a sort of wall when put in choke points as creeps only try for so long to go in one direction before they turn around. We have noticed very often that creeps will bounce back and forth between these auras and not advance when set up as mentioned above.
Next, you will want a lightning aura right after the enrage auras. Anything that makes it through the enrage aura is already going to have lower than max health and this will take out most of them. This will kill any of the fire immune creeps that just pass through the enrage. These auras are also useful where creep flow is minimal. For the Deeper Well that is top paths where only one or two creeps will appear and go for the crystal. Combination on these low creep paths with an ensnare aura helps but you should consider putting down two auras just to be safe.
Any area that is not covered by the top two paragraphs should be After those areas and in path ways leading to your crystal. I highly suggest filling up these areas with as many ensnare auras as you have the defense points left for. Consider a weaken or healing aura near the crystal where you may have to get up close and personal with a creep. And while it seems odd, I also recommend having one path always have one less ensnare aura than the others. This will be your Go To path when you have creeps coming in multiple directions (Note that this should be a minimal number of incoming creeps). This makes it very clear which one to clear out first and can often help by getting the creep to you sooner.
That pretty much sums up how I/We currently play Monk Only.
EDIT: Application of the things detailed above.
The next 2 levels add in another choke point on the last 3 hallways towards the crystal. Simply repeat the pattern above for the extra path to the crystal. Start with the enrage so that when things get fed up they will try and go to one of the other paths. For the third level (Magus Quarters) you have to take out the flying creeps. You -can- set an aura that they will run into but its not a guaranteed placement like for the ground creeps.
When dealing with the alchemy lab (this is where we developed these strats actually) you use two enrage auras as close to the lower entrances as possible with the lightning ones right after it followed by another slow. Then up where the stairs come down from the main entrance put an enrage on each winding stair case down. This keeps them in the enrage as long as possible. In the center of the flat areas is where you put the slow and electric auras, then where things converge again consider another enrage or a slow. This keeps you in the center ready to take on the few lightning immune that make it through the enrage auras. What you should notice though is that the lower entrance creeps will be caught between the enrage turrets and wander back and forth.
I am not saying you will for sure be able to make it through the entire game as a monk and I can almost guarantee you will not be able to do it on insane, but the key is to find the choke points where creeps meet and enrage them so they will then turn around and go the other way to the other enrage aura. Some times these areas are very near the crystals like the 2nd and 3rd level, other times they are near the creep entrances like levels 1 and 4. I have not gotten to Ramparts yet, but I would imagine the lower entrances, at the bottom of each narrow stair case is the best place with the enrage then other auras, Up in the mid section there is a point where the creeps will split for the side stairs on the lower and upper crystal which is the place for the enrages there then the front steps, same thing. You will again need to take care of the flying creeps most likely by hand.
I can not stress Map Awareness (learn to play holding down the shift key if you can) and the use of a multi-shot ranged weapon.. My level 24 monk, for example, has a 4 shot 102 damaging weapon. Takes out most things on hard in one shot if I am up close for all four bolts to hit them. Just cause I have a ranged weapon does not mean I use it at ranged, it just attacks faster than a swing.
James' answer is spot on in most respects (I soloed two monks up to level 20 at least, one on open and then again on ranked, my 74 monk is my main). I would amend it in the following ways.
First, I would argue that melee is the way to go with a monk. You want to be taking advantage of hero boost by the time level 20 rolls around, and it's an ability that heals you while doubling your DPS, basically. The perfect move for a melee-oriented build. That way instead of having to decide between kicking more ass or stoping to heal, you can heal while you kick even more ass. Also you'll need to be close to your allies to give them the boost too, which means you'll want to be up in the action rather than firing from afar. But ultimately, make sure you can do both kinds of attack with at least passable effectiveness. Don't accept any weapon that nerfs one completely.
When you play a monk, you learn to turtle. In the first wave where there are little to no enemies, you should always try to have a strength drain OVER all the crystals to minimize the melee damage it takes. This gives you more response time to get there if you see the "a crystal is under attack!' message hit your screen, and if you're a soloing monk, trust me you'll be seeing it at least occasionally. If your aura radius is not great enough to give you full coverage around all sides of the crystal however, consider ignoring this peice of advice, especially if there is more than one route to the crystal in question. You dont want to waste any more DUs on strength drain than one per crystal.
Two auras that go together - ensnare and electric. Electric aura is a DoT (damage over time). Ensnare just plain gives it more time, they complete one another and a good spot for one is a good spot for the other, at least if your goal is the kill things. But what about the electrically immune enemies?
Well, the enrage aura is (IMO) the best defense in the game for a reason. That reason is that unlike literally all other defenses, its effectiveness INCREASES the more enemies that hit it at the same time, rather than decreasing by losing health or charges. Also, it lasts a long freaking time for what it does. Plus it's just lulzy to watch. One thing I will warn is NOT to put ensnare auras and enrage auras in the same place. The ensnare will go down, assuming the enrage is doing it's job right. Ensnares do not have the life, even in their upgraded state, to survive the kind of "use" that enrages subject them to, and even if that wasn't the case, why would you want to slow down the enemies that are killing one another?
So my usual setup for "walling" a chokepoint is one enrage followed by an elec-slow combo only barely overlapping the back of it (so that the enemies who might otherwise be leaking out of the enrage are slowed down only momentarily so their speedier counterparts can still hit them in the back before they leave). This in effect stops ALL enemies except for the few lightning immune ones who manage to escape the enrage. Those are the ones you'll hear about when they attempt to ping the crystal in their strength-drained state. Mozy on over and take them out at that point, at your leisure.
Put this setup on all and only those chokepoints closest to the crystal, making sure those chokepoints are still out of arrow shooting range. Then throw one healing aura down in the center of where you plan to patrol and call it a day.
You can certainly solo the entire game as a monk if you so wanted to. Might take some time, but even on NM, if your monk was geared enough, you could do it, without any other class's towers.
Remember you want as MANY ENEMIES AS POSSIBLE to hit your enrages at the same time. Nothing should come before enrages to stop them from getting there and nothing should try to kill the ones that ARE there. Every time I pass one I check for leaks but then say, "'Tis a grand melee, leave it alone!" Then I come back at the end of the wave and clean up for my Lord Award, once it's fully ripe for the picking.
The question is, what are you trying to accomplish by playing solo single class character? Dungeon defenders was designed as a co op game, and no matter how godly you are, you will be hard pressed to beat the later maps and challenges on solo. Especially on maps where the crystals are scattered all around the map with multiple spawning points.
The only way I know of to solo effectively, is to play multiple classes, such as using a squire barricades to wall of certain areas to hold enemies back while you get the time to handle them. There is simply no way for a single monk, or a single class type to solo the entire game except possibly the easiest difficulty. And even then, soloing with multiple classes is still quite the challenge.
tl; dr - You can't solo the entire game as a single monk. Play with others or make multiple classes.
the strength drain mixed with electric is good also, it can demolish all enemy types, put an ensnare on top of that, and no enemies should get through well. put enrage auras before and the enemies that flow into the tri-aura are already weak enough to pick off. the problem is that it takes lots of defensive units. hunt down the choke points, and use the least amount possible. i think the most choke points any map has is 6. not including DLC
I would reccomend to play the Monk with an Apprentice. Why? Well, the apprentice magic walls remove enemies elemental resistance and that allows auras to take effect. Just build a wall on a choke, with decent hp and put a ensare, electric, strenght drain and maybe enrage in front of magic wall. Behind the wall place aprentice towers and a heal aura where you can retreat to fight at range while healing. Magic walls greatly benefit auras. Do not underestimate magic walls.