You have a limited amount of time to play DeathMatch. So if you want to win and win fast, you gotta think outside the box in order to defeat your opponent.
On every turn you make, you have to keep your opponent guessing. scare him/her by doing the unexpected. Which brings me to this...
DON'T ALWAYS GO FOR A K.O! K.O's are pretty worthless in DeathMatch and you should'nt waste your time trying to. just bring your opponent's life low enough so you can win. Big hitters and DR's will do the trick.
I suggest using a clan you really like, or are suited too (same thing i guess)
Whether its t1 DM or t2 DM, try to think of getting ahead of your opponent. So like what AKG said, having DR wont hurt if you have them.
It is all about speed in DeathMatches. Once I was neck and neck in wins with the 1st place guy because we both had 6. Probably the last few minutes of the match he was about to get a tie and a loss while I got a loss. So it ended up 6/0/1 for me and 6/1/1 for him and if my opponent didn't take so long to make his move I could have gotten into another game before it was over.
I think it is kinda stupid that your opponents can determine whether or not you win the deathmatch since they can take their time each round.