Gaara and the Wind girl seem to work fine together, especially if you get him to 3* for that wind user buff and shield. Pair him up with Kankuro and another wind person and you'll do fine, though you will have trouble against all fire teams.
There is no "best build" today i faced a player same lvl as me, same ninjas as mines an i beat him and at the final i was with 48% health, so you cant build "best" pvp character i think
Actually, that can be due to a lot of things, equips-magentas-other stuff or just plain and simple good luck with crits.