...I need a sound effect for essentially an earthquake.
BKKRRRRRMM... is the best I got.
Also I need a sound effect for what would be the equivalent of, I guess, The Hulk punching through a giant metal door.
WHPANK! or BWHAM!, perhaps?
...I'm just going to use Unsound Effects
even for things that have widely accepted onomatopoeia.
That's a good idea for a comedy. May lead to Narm in a more serious work, though.
Personally, I don't like using onomatopoeia that everyone's familiar with, like "Pow" "Splat" or "Ka-Boom". It's much more fun to come up with your own. Heck, I used "Whcrunch" in a comic earlier today.