isn't ice beam preferred over fire blast though?
it ohkos gliscor without quiet/evs, dragonite can survive twice if it invests in sp def and boah doesnt invest at all in spatt evs.
what else is there?
The choice between Fire Blast and Ice Beam is pretty much based on what your team needs. If you can handle the threats tehy mentioned with some other Pokemon in your team, then you can go with Ice Beam to take care of Gliscor or Landorus or something. However, if you can't handle Pokemon like Skarmory and Scizor with other Pokemon in your team, then using Fire Blast would be better.
in which case, why would you go quiet over brave for something so specific and unnecessary?
tyranitar is a very good pokemon, but people need to stop running that crap sassy set with no attack evs, run a non speed hindering speed nature and 50 evs or so to creep scizor, then roast it for the ohko with fireblast as they go for the uturn (which they always do) stuff like this just leaves the opponent stunned and is a great way to break the annoying scizor rotom-w uturn cycle whilst setting up your reuniclus for a trick room sweep.
Conclusion, standard sassy mixtar sucks
I've been racking my brain for a lead Deoxys-S counter, but the only thing that comes to mind is a very specialized lead Tyranitar:
Tyranitar @ Focus Sash
Sand Stream | Sassy (+SDef, -Spd)
240 HP / 208 Atk / 60 SDef
- Crunch
- Brick Break
- Stone Edge
- Stealth Rock
208 Atk to guarantee the Crunch OHKO after Sand (and honestly, the 32 EVs from 208 to 252 provide a 1 HP difference in power, so they're not needed), 60 SDef and a +SDef nature hits a jump point in Sand, and the rest go in HP to do more tanking. Dual Screens leads get 2HKO'd by Crunch, just to have their screens smashed on the next turn; Stone Edge for horribly inaccurate coverage; and Focus Sash to always guarantee Rocks in the event that you're not facing a Deo-S. Sassy + 17 Speed IVs to underspeed Politoed in case you're playing a weather team and want to keep TTar alive.
Sadly, this leaves you as a sitting duck against alot else :(
A Quiet nature allows Tyranitar to reach levels in Special Attack that would let him take care of things like Skarmory or other Steel-type Pokemon that could otherwise potentially wall the TyraniBoah set. You don't want to be walled when you're under a Substitute and ready to roll, so a Quiet nature would be better. However, if you want to use a Brave nature because it would be better with your team or something, then feel free to do so.