My guys end up fighting the front lines while skeleton crossbow enemies pile up until they become undefeatable. How can I stop the enemy archers from living long enough to form an undefeatable platoon?
I managed to get an infinite game going for over an hour, before I got bored and killed myself. I had max level swordsmen and spearmen, and spammed them constantly. I kept a max level wiseman up for a little healing, and used arrow & catapult volley on cool-down. When I had extra money i'd buy an archer (level 1) or arthur (level 4). I had max Ash upgrades, but only smithy (level 1) and wall archers (level 1) for castle upgrades.
Keep Ash back and only use his ranged attack. Eventually your troops will push the enemy all the way to the entrance gates, and hold them there forever. You have to run in and out to grab gold/iron in between Evil Ash's swings, but its manageable. I raked in about 200k gold in a little over an hour.
Basically the damage output of the spearmen group was so huge that it would kill the archers before they could do any real damage. The swordsmen group could survive a few hits from Evil Ash, and the Wiseman helped stem the losses here.
The only problem with this technique is that there are so many units onscreen that the game slows to a crawl, and its very hard to play. I suspect you could ignore the archers to get a little extra frame-rate.
I found a few new ways to maintain an infinite game. Unfortunately, at some point the frame-rate limits how many units you can build (or the game crashes). Basically I build until the frame-rate is barely playable, then replenish lost units as they die. I have managed 130 million in endless before getting bored and ending it. (500k gold from this).
I do not build any units until the smithy and iron are maxed. I build Arthur and Wiseman first, and start piling up my tank and damage units behind them. I dart in and out to collect resources at first, but eventually I park Ash on the right edge of the screen to auto collect. The key is getting your damage high enough that the enemy units, in particular Evil Ash, die before they can kill Ash. The Wiseman is necessary for this.
Ash abilities: I've found that the two volleys (arrow and catapult) are the most useful, as they are area effect and set their targets on fire. Once on fire, units will burn quickly until dead. I use them to take out groups of enemy archers from beyond their range, preferably before they massacre my front line.
Castle upgrades: With a Pit, you can easily turn a strong enemy push back the way it came. The castle catapult will knock enemies in, as will your catapult volley.
The most entertaining method I've come up with involves spamming archers (with a torchboy) and spearmen, using Arthur and Wiseman to tank. It is necessary to use volleys to kill enemy archers every time they spawn. When Arthur dies, you get pushed back to the Pit, where you can re-establish your line by using knock-backs to toss enemies into the Pit while your new units pile up.
The most effective method uses expensive, high damage units so that you can build more of them before the frame-rate interferes with your game. Use armored guards, horsemen, Wiseman, and Arthur. Build up around 20 or so armored guards, then build horsemen until the frame-rate is borderline unplayable. The damage is very high, and Evil Ash barely lives for 1 swing of his sword. I was able to leave the game unattended for half an hour without any units dying.
My combination is wiseman with Arthur and armored guards for tanking and for damage horsemen and spearmen as these 2 have the highest damage output plus they can hit from bit behind. For spells i use the slmagic words and the deathcoaster. After a sertain point Evil Ash hits only once and then he dies, sometimes he dies before even managing to hit. I have a game going for a while and when i decided to finish it i got 1.000.000 gold which was more than enought to upgrade everything to max.