How would flamethrower help with spacing?
ShellShifter: Retreating nairs, bairs, and rock smashes, if executed safely, work well towards controlling space even against faster opponents. However, it's not enough to perform those things, what really holds most zoning tactics down is the threat of punishment if the opponent tries to get in your "zone". Charizard's grabbing range means a grab is the most imminent threat for anyone who tries to enter Charizard's space, so remember that you must always keep that threat present. On the ground, jabs and ftilts offer decent protection as well. Use dsmash for opponents who cross you up and land behind you, since Charizard's back is often perceived as his weak side, even though he has decent options.