Competitive assistance - which mons should I use in my box?

As the title says, I'm looking to create a synergistic team. I'm really struggling against stall teams, especially mons with flash fire, water absorb, and immunities to ground type. So heatran, gliscor, ferrothorn, etc. With the mons I have, I have found those most success with garchomp, quagmire, chansey, and some success with serperior and a nuking set on porygon z.

Admittedly I'm not too great at team building, so again any help is greatly appreciated!

Edited July 5Jul 5 by Zaldooo