Except you're never going to ever keep a Sub up against a Shadow Claw unless they're burned and you're physically defensive, so you're already fighting a losing battle- 50% of the time you're going to outspeed the first turn and waste 25% of your health, which means you can't set up subs any more because if you sub again it'll either go last, in which case you'll fail to set up sub because Shadow Claw will ruin you, or you'll go first, in which case its just the same situation as before, but you're just playing against the odds. Even if you go last on the first turn at best you can manage a roughly even HP exchange, because nobody is going to stay in once Cursed.Why on earth would you ever curse first before Substituting.................................
if you are not in danger of getting OHKO'd,you are guaranteed to get a substitute up provided you move last. From there you can protect to get the sitrus regen and (Majority of the time) get back to over 50% life and sometimes back to 70 ish, which is enough to use curse.
from there it's as simple as protect -> Sub mashing/Phantom Force(if your sub is still up or you're not taking more than 50% damage and have the health) -> Protect again to get sitrus regen if needed and stall till curse kills. if you are in danger of getting KO'd by some super powerful super effective move and your sub is already down and you didn't spam it again to annoy them, You protect for curse to take the 1/4 and then YOU SWITCH OUT. People are starting to carry taunt users in the higher ratings, making Curse Stall alot more viable and useful compared to sub seed with WoW because you can actually stall for 2 turns with PF if they switch a taunter in on you. Sub/Seed/WoW is too predictable, easily dealt with, and hardly ever sets up the situation where you can bring in CB Scizor/Talon flame and clean up the match late game.
as someone who has been using trevenant both in game and on simulator in practically every team, Curse/Protect/Substitute/Phantom Force is the superior set
And this is presuming they run Shadow Claw, Phantom Force rips you a new one.
Quite honestly though in any Trevenant vs Trevenant scenario I wouldn't feel safe no matter what my set is, safest action is to switch out.