As you have a Charizard X your movepool should be physical only, because of Dragon Dance and Tough Claws, so Focus Blast is out of the question. If you want a Fighting Type move, here are two candidates.
- Brick Break
- Power-Up Punch
Power-Up Punch is a weak move (40BP) however increases your Attack by one stage after using, and can boost your Attack to astronomical levels. Brick Break can destroy the opponents Light Screen or Reflect, and has more power, but offers no boost other than that.
Here are some damage calculators which show the devastation Charizard can cause
+1 252+ Atk Tough Claws Mega Charizard X Power-Up Punch vs. 0 HP / 0 Def Mega Tyranitar: 264-312 (77.4 - 91.4%) -- guaranteed 2HKO
+1 252+ Atk Tough Claws Mega Charizard X Power-Up Punch vs. 252 HP / 252 Def Mega Tyranitar: 224-264 (55.4 - 65.3%) -- guaranteed 2HKO
>252+ Atk Tough Claws Mega Charizard X Brick Break vs. 252 HP / 252 Def Mega Tyranitar: 280-332 (69.3 - 82.1%) -- guaranteed 2HKO
>252+ Atk Tough Claws Mega Charizard X Brick Break vs. 0 HP / 0 Def Mega Tyranitar: 332-392 (97.3 - 114.9%) -- 81.3% chance to OHKO
So, against Mega Tyranitar, a +1 Power-Up Punch will always 2HKO, and a neutral Brick Break will always 2HKO or even get a 1HKO. So, in summary, use Power-Up Punch if you want a move that can boost your attack so another move can sweep (Flare Blitz etc) and use Brick Break for an easy to use move that has a neat bonus, without boosts.
Edit: That set isn't very efficient for competitive. Fly is a horrible move that receives no STAB and takes two turns to execute, meaning someone can set up and kill Charizard. It also is a physical move and your Charizard is clearly special based, and even that is a problem. Charizard X is WAY more efficient as a physical attacker, and using one with those moves ain't going to get you far. Try this set.
Charizard @ Charizardite X
Ability: Blaze > Tough Claws
EVs: 252 Att / 252 Spe / 252 HP
- Dragon Dance
- Dragon Claw
- Flare Blitz
- Brick Break / Power-Up Punch (read previous area)
Pretty simple stuff, set up with DD and sweep the opposing team.