this must be added, but Big Bang is kinda like After Math isn't it? I'm assuming it's a bit stronger is all, but I propose something a little different.
I suggest: A. Keep the ability Big Bang, but make it so that when Mega Golem faints, it always does 50 HP damage to the opponent and leaves them with a new status condition, a harsh burn. Harsh burns do more damage over time like toxic, and cut attack damage in half, but they can be put out with water and ice type moves, abilities, or weather conditions.
B. Give it a new ability I like to call "Magma Core" which makes it so that any pokemon that physically damages Mega Golem always gets a harsh burn (unless they are immune to it via item, ability, type, or weather condition)
I like my first idea, but it kinda seems OP. If harsh burns aren't added (which I doubt they will add) then Big Bang should just leave the opponent with a normal burn.
You're right about the harsh burn being a little overpowered. I think it should take away a fraction of damage rather than just the set 50. Maybe 1/6th of the opponent pokemon's maximum hp? 1/7th or 1/8th for types resistant to fire types? I also think leaving the opponent with a burn is a good idea.