EA SPORTS UFC 3

In order to submit an opponent you must first take the fight to the ground ( + ). After the fight is on the ground try to get in an advantageous position and every once in a while hit and check if a submission is available. If one is, select the button combo associated with it and you will enter the submission minigame. If you don't know how to complete this minigame I highly recommend doing the submission skills trainer as practice. Basically flick in the same direction you see your opponent doing.

I cannot confirm this works on solo challenges or solo championships on Ultimate Team. Did the game months ago and I can't remember.

But other multiplayer achievements works on these modes so probably this one is working as well.

But for all these multiplayer achievements, best solution is to find a friend (Social Club on Xbox works perfect), invite him on a private match and boost all these achievements. In just 2 minutes you will get everything and the next match your friend will get anything.