Iron citron has a ton of hp but is really slow and only shoots one round per clip. It also takes him awhile to reload, unless he is maxed out. A maxed out iron citron will more than likely have the legendary meter upgrade and two reload upgrades applied. If so the double reload upgrades make a huge difference.
They generally stick to the back and try to knock off easy targets so they can fill there meter. Once they're in iron mod, watch out. He gets a huge speed increase and an extra 4 rounds in his clip for a total of 5 shoots per clip. Also has a fast fire rate. A direct hit does 50 damage, so he can deal 250 damage real fast.
Best bet is to never let him get in iron mod. Sneak up on him from the side as an allstar and start shooting. Do as much damage as you can and then imp punt him before he balls up and runs away.
Next best thing is to use the parrot from the pirates. You might not take him out with the parrot but at least you can clear him out from his camping spot and force him into the battle.