credit for this goes to Xbox 360 achievements for this solution
Make a character with low attack, high defense, high hp
For skills, have Hp Drain A and/or Hp burst
Play TOS ascension Floor 24 and just let them attack you
When you are low on health, finish them off 1 by 1 to regain health, or use Hp burst
Keep the last guy at low Hp so that you can win quickly because I got this achievement with 4 seconds left
Alternatively, go online and do this in a player match with a friend.
extra: This method worked out for me better than JAGMAs method since the CPUs on floor 24 seem to attack you alot more
It personally took me 3 tries to get this achievement