This video guide will explain to you how to do it solo on heroic, and maybe a bit better than i can here but, basically you're going to want to get your whole fire team Incineration Cannons before you enter the Warden battle because otherwise you wont do anything other than be b8. You can find two Incineration Cannons after your first encounter with the knights in Mission 14: The Breaking which will be pretty early on, before you ride any lift pads. After you kill the knights tell your team to pick up their weapons and you got two out of four.
The next two you can find are after riding the first lift, you'll enter a room where you have to fight a bunch of Crawlers and two Splinter Turret Soldier Captain's, at either sides of the room there will be ramps, and at the top of the ramps there will be two more Incineration Cannons at the top of the ramps. Give them to your allies in the case that they died, or just give one and take the other if you wish.
You can find the last three Incineration Cannons after the second lift pad, about half way between the last pad and the boss fight, there will be a giant glass window in the middle of the room with an incineration cannon in front of it, take it for yourself but don't use it until the boss fight. If you need any more IC's because of your teammates dying you can pick up two more after killing the Knights in the area with the snipers.
When you get to the actual fight you want to target one of the Wardens and try to kill him a quickly as possible using charged IC shots (to the back if possible). Once you kill one of them, run around the outside of the arena to get a checkpoint. Then alternate between the two wardens activating their recovery animation by telling your allies to damage the one you need to activate next. After you've activated each maybe like 4-5 times make sure you have a full ammo IC, you'll find one on either side of the arena and two under the bridge the warden breaks when he spawns. If you feel like your close to killing them tell your teammates to target one, (assuming at least one is still alive) while hurting the one, you didn't target, the same amount your teammates are doing. When one dies you have maybe 1-2 seconds to kill the other one for the achievement to pop, if one dies and you don't think you can kill the second one in time, restart the last checkpoint immediately.
Don't even try to kill the next one, unless you know you can kill him in time, otherwise you will pop a checkpoint that you cant come back from, and you'll have to restart the level. If you think you were close enough but not 100% sure, don't wait for the achievement to pop try again, because 99 times out of 100 it wont pop that delayed. Even if it does, then you'll get it as your trying it again, so better safe then sorry. Check the video out for a visual representation of what i'm talking about.