Every time I get crippled from AA fire or cannon fire, or a missile that exploded due to it's proximity fuse, I always bail out cause no matter what I do, I always explode when crash landing.

So, is it even possible for a crippled plane to land on a flat, grassy field in lock on?

Yes---but difficult

anyone actually succeed want to give a few pointers?

little faster than normal, slightly higher AoA than normal. Makes it easier to e-a-s-e the plane to the ground instead of impacting.. The Advanced flight model in FC allows for a lot more realism in this regard. I think you can get away with belly landing on a runway in simplified fight model planes fairly easily- I think the terrain landings have to be knats' *** perfect or you blow up..

My wheels up landing was on flat terrain. But I am not sure I could do it again or not.

my usual procedure as i learned it with IL2
is to touch down as gentle and slow as possible

that means full flaps while flying very low. everytime the plane wants to decent you raise the nose a bit more so when you finally stall or touch down you are going as slow as possible.
nose high is important aswell.

no wheels!!!

Try to land on a straight piece of road if you can find one close enough to you.....