So I play this game casually with a bunch of friends together, and I'm going to be honest, none of us are particularly that good. One of my friends started experimenting with Isaac and found that spamming that mine thing and his square gun thing in a corner work really well together. What is a good counter for this? I really want to beat his arse into the ground.
Patches the Hyena (Dark/Demon's Souls) for All-Stars! His level three could have the player leave the match, enter a random match, and get one-hit-one-kill's on everybody! xD
You have some option depending on the character you are playing and the map. If its the map, you can leave im alone in his corner so if he cant hit anybody, no ap for him and he will stop camping behing his mine. You can also jump behind him and grab him out of his corner while hes shooting, the neutral square shooting move is easily punishable. You can also destroy the mine, theres a lot of move that destroy the mine without you getting hit by the mine which ppl dont realize. You can always use ranged character and throw projectile, its a good way too.
And the RUN DOOM post is funny, but not very helping lol. Hope i helped you out.
Other: Kratos, Parappa, Heihachi & Sly
English is not my main language, so im sorry about my mistakes.
Counter-pick with Zeus and spam his super armor electricity, preferably from above until he switches strategy.
Edit: Or with Kratos and destroy his mine with your square moves, and keep him on the ground with a.d.1.
I find Sir Dan extremely effective against Isaac's detonator. Use his fire (down circle) in the air, and it destroys it, plus for period of time, as long as the fire is up, he can't place another down there. The axe (neutral triangle) and skull (forward circle) also do a decent job at destroying the detonator. Yet another thing to do is once you are above it and coming down, do down square and swipe at it. Not only does this destroy the mine, but it knocks him down if he's in the way, and you can work from there.