Sure...in Tag I used Ein and Christie...
I play with one character at a time and don't switch until they hit 50% damage...then I switch out and get the opponent close to death...switch back in and finish the enemy off to get the boost in life...then return to the other characther so the computer does not get used to the attacks I throw...
That is a big part...vary your attacks...the CPU will see your patterns after a while and crush you.
Try and pick up the items dropped...mainly the food items as they restore health...and be weary of the characters that give you the most trouble.
Also be careful around the 20, 40, 60, 80 and 100 mark...the CPU seems to tighten up around there so make sure you have the character with the most life in at all times around those levels...
The only other thing is practice...I got the game the day it came out and just now got to 100...also yesterday I got 'Mad Cat Skills' in Geomitry Wars