you can change single attack to 1 round and still get the achievement... AFAIK you can also change the time limits..
But yeah you just need to be agressive, do power moves constantly your goal is to remove as much health as possible as fast as possible..
Personally I say avoid long combos I typically pick characters like EIN and just to 1 or 2 hit power moves over and over. Someone like Lisa or Tina are really bad choices because they waste a lot of time to execute their moves. Also avoid throw moves unless it's a tag combo and do everything you can to keep your opponents from taging out.
EIN, Leon, Baymen, Nicole, Hyabusa, Haiate, etc. are all good choices for this mode. You can get by with some of the other players but you have to be VERY selective with the moves you perform.
Basically you've got 5 matches... which means you only have 30 seconds per match... the first 3 match you NEED to come in WELL under 30 seconds or you're hosed because the later matches have increased difficulty (so your opponent is more likely to counter/block and fight smarter against purely offensive tactics) and will most likely take longer. Just keep a mental note of how far ahead or behind you are, and pay attension to how much time moves take to perform... stick to moves that take a lot of damage in a short amount of time and cheese them.
good luck... this one took me a while