-Flying type, so that takes away that Ground weakness.
-Can learn Earthquake, which covers 4x Rock Weakness.
-Pretty good stats
-4x weakness to Rock
-Doesn't evolve as early. (level 16, then level 36)
-Not the biggest movepool
Focus on Attack and Special Attack.
Attack EV spread: http://pokemondb.net/ev/attack
Special Attack EV spread: http://pokemondb.net/ev/spatk
I guess you could also focus on Defense, but not with this Charizard. With another Charizard, though.
Defense EV spread: http://pokemondb.net/ev/defense
Charizard @ Leftovers
EVs: 252 Attack / 252 Special Attack / 4 HP
Naive Nature (+Spe, -SpD)
Earthquake -(Do I really need to explain it?)
Fly/Wing Attack -(STAB)
Double Team -(Helps evade)
Flamethrower- learns at level 34.
Earthquake- TM 26
Fly/WingAttack- Fly is HM 2, and Wing Attack is learnt at level 36.
Double Team- TM 32.
Hope This Helped :D