Wait, hold up. Why use heatwave in OU? You don't need to hit more than one opponent unless you're playing Doubles or VGC. If you're really worried about accuracy, then go with Flamethrower. It has better accuracy and only a slight power drop. Otherwise, Fire Blast will get you KOs much easier.
As for Scald vs Hydro Pump, I had this dilemma for my Greninja as well. I went with Surf in the end, because I almost never hit Hydro Pumps. With Scald, Burn is good, but sometimes it won't get the OHKO that Surf can get. And Greninja is a really frail Poke, so even a burned Physical Attacker can usually take it out in one hit. Especially if it runs Life Orb.
Air Slash is AWESOME btw. That flinch can come through Clutch a lot.
And one more thing. If you really want to go with a Toxic Stall set on your Gliscor, then I would recommend going with this set.
Gliscor @ Toxic Orb
Ability: Poison Heal
EVs: 252 HP / 40 Def / 216 Spe - The HP and Defense are Self Explanatory. The 216 Spe allows Gliscor to outspeed +nature Base 90 Pokes. This allows Gliscor to get off a Toxic first and also Subtitute before they attack.
Jolly Nature - Gliscor doesn't really need more Defense, it really only needs to tank one hit in order to Toxic, and then use Speed to stall
- Toxic - Self Explanatory
- Earthquake - EQ over Facade because EQ is Super Effective against both Steel and Poison types, which are immune to Toxic
- Protect - For Stall
- Substitute - For Stall
