Tretsujin

Best answer as chosen by Asker / Ratings
You can melee as any character,
To melee, go directly in front of the enemy of your choice (or any side I believe will work) and press the right trigger.
The character should then do a melee technique such as Headbut, Slap, Throw etc.
However, remember, when you are doing a melee attack you still take damage from enemys around you.

If you need more help, my gamertag is the same as my Username here (Tretsujin) so just send a message / friend request.
Hope this helped.