How viable will Tyrantar be for Deoxys?

Tyranitar is super tanky, no worries. According to PB, it's the undisputed supreme counter against the ZH sets, but even against the C sets it's in top 3 TTW and 3rd, 4th or 9th (Zap Cannon) in TDO. It will perform great regardless of the Deoxys moveset.

If you can identify from the autoselect that the boss is running Counter, you may want to bring in more Gengars and Mewtwos if you want to optimize. But remember also that this is an EX raid and a really squishy boss, most of the time you will likely only need your first 1-2 attackers.

Counter will hurt for sure, but your bigger issue is if that's paired with Zap Cannon. Tyranitar has roughly the same DPS/TDO against the other 5 move combos, C/ZC is the only outlier. Either way it's still a top tier counter, a team of 6 alone can take out well over half of Deoxys's HP with varying levels of reentry.

It doesn't matter because the moves will likely just be switched around anyway. I seriously doubt that will be Deoxys' final moveset.

I think the bigger issue could be if he has different forms. All of the forms should be relatively easy to beat. The defense form could be a nightmare though. His defense while also not dealing as much damage could make him more a headache, let alone going against a potential Counter+Zap Cannon moveset. Will be interesting but since most EX raids host 15+ around here it should still be easy