I believe there are a number of strategies you can employ to beat Plantera, but the following one has served me well.
Since you've beaten the three mechanical bosses (a requirement as of 1.2.3 to get Plantera to spawn), you should have several Hallowed Bars. Use them to make the Hallowed Armor if you don't already have it. If you're adventurous, plunder the underground Jungle and mine for Chlorophyte Ore to make the Chlorophyte armor. It takes a lot of Chlorophyte ore to make the full armor set, but it's worth it in my opinion. Note that you'll need the appropriate pickaxe or drill to mine Chlorophyte (both of which are crafted with hallowed bars). Farming a mechanical boss or two again may be necessary to get enough bars (the Destroyer is my favorite one to farm).
If you do plunder the underground dungeon, keep on the lookout for Life Fruit. The permanent boost to your health that each one provides is always a good thing!
As far as weaponry goes, I like using the Megashark with crystal bullets. A full hallowed armor set with the hallowed helmet yields a full-set bonus granting a 25% chance to not consume ammo. This is great when coupled with the Megashark! The Chlorophyte helmet has a similar bonus (though only a 20% chance not to consume ammo).
Accessories are up to you, but I like rolling with the following:
- Charm of Myths: absolutely necessary as it gives life regen and a 15-second cooldown on health potions
- Cross Necklace: increases your length of invincibility after being hit (which means you get hit less!)
- Leaf Wings: These cost a fortune (1 platinum coin), but wings of any type are much better than the rocket boots / balloon
- Obsidian Shield: Prevents knockback and +2 defense
- Spectre Boots: Gives me the ability to run fast (wings override the rocket boot portion of this accessory, so Hermes boots will be just as fine)
I don't pay much attention to the stats on my accessories, but having some additional defense on a few of them never hurts.
Make sure you have plenty of potions:
- Greater healing potions are ideal
- Iron skin potions
- Thorns potions
- Swiftness potions
- Hunter potions (a nice to have ... not necessary)
- Regeneration potions are very helpful
- Bowls of soup (and other similar items) are also useful
I like mining out an "arena" in the jungle where I can do battle. I essentially make a large square that I can fly around in (this is where the wings come in handy), walling in passages as necessary to prevent other enemies from wandering in (or, more importantly, me from stumbling into those passages by accident during the heat of battle). You'll need a few things for your arena:
- A campfire is essential (I usually place mine at the bottom of the arena)
- A couple of heart lanterns (in opposite corners of the square) are essential
- A small pool of honey can be useful
- A heart statue (or two), wired up to 1-second timers are very useful
- Spawn the nurse into a protected, yet easy to reach area for instant healing.
Once you've got the arena built, make sure you know how to get from the bulb to the arena (if your arena isn't located immediately near the bulb), and lure Plantera there once you've broken the bulb. As you lure it to your arena, you may have to move slow enough to prevent Plantera from de-spawning (I seem to have had that problem in the past). Once you're in the arena, it simply becomes a matter of flying around the arena, kiting Plantera as you unload your Megashark. Phase one should be relatively easy; just avoid Plantera as you fly and shoot. Phase two becomes a little harder, and switching to a strong melee weapon may be necessary from time to time to keep the smaller tendrils from swarming you. Regardless, keep kiting, keep quaffing health potions, and keep picking up hearts from the heart statue to stay alive. I assure you that Plantera can be solo'ed ... if I can do it, anyone can. :-)