Here is how I fight them when I first get to them. I'll do my best to describe in text what would take two minutes to show on screen.

When you first enter that area there is a low structure in front of you, in between you and the canon. Fight the midget from there.

If you have a sniper you can get a shot or two off on him before he closes with you. If you are super good with that sniper and/or leveled up a bit you can kill him really fast. Don't worry about that to start but know that it will become an option as you improve.

Anyways, that midget takes the same route to you each time so you can get some muscle memory with fighting him. Stay behind the tall, center part of that structure and you'll be safe from the canon shots. Go for headshots on the little guy, go for crits, and otherwise just hide from the canon by keeping that structure in the way.

When the little one is down you can close with the canon. Run on a tangent to it to avoid most hits and close with it as best you can. Once you get right up on it then you can just circle around it, staying behind it, killing the psychos as they come out and taking shots at the canon in between.

If you want, you can actually control where the canon moves and where you stand in relation to it by moving from one side of the back of it to the other side - the canon will always rotate the 'shortest' direction to reach you. You can use that to keep the canon pointed out towards the large area of the map and yourself on the small ledge behind it. That will give the psychos a very small route to you and make it so that you always know what direction they are coming from and when they will arrive.

Once you get the canon blowed up you can move around to the other side and kill the last guy in a straight up fight.

If/when you get good at that, it is a great area to farm up some levels before moving on to Flynt. I will often get several levels from those two guys before I move on. A nifty side-effect is that you will sometimes get a Legendary grenade from them.