Er, that would only be true if Mediguard was permanently active. Of course, it is not. Snow(/Fang) needs to regain the ATB he uses on Mediguard at some point. Needless to say, this is when the opponent will attack. So during that time, only Snow's normal sentinel reduction applies, which is 44%. Deathward, similarly, will not apply during these attacks, which are where most of the reduction has. So right away, you only need 36% damage to punch through Mediguard: the sentinel regains 20% health, then 56% of 36% is >20.
But additionally, as you note, Mediguard is
slow. And not just kinda slow. Each Mediguard (out of the five) takes about 3-4 seconds (I can link a YouTube video if you doubt this claim) while normal actions take closer to 0.5 seconds (per Tal's notes in the stat topic... looking things up some actions are slightly slower, but everything is still much faster than 1 second). Toss in the 4.5 seconds it takes to charge in the ATB, and a "normal" round is about 7-9 seconds, while a 5x Mediguard one takes 20-25! At the very, very least, you should give anyone fighting an FF13 sentinel two turns to the sentinel's one. Granted, the second turn will hit Mediguard itself, but that's still one more attack at 20% power (assuming Snow uses Paladin). So in total, the would-be sentinel slayer gets in one attack at 56% and one attack at 20%, for a total of 76%... you only need to do 27% to the sentinel to do that, or >33% to Snow due to his HP. If Deathward manages to apply (it won't always), then it's 56+15 = 71% against 20% healing, which requires >29% to the sentinel, so >36% to average when that sentinel is Snow. And that's ignoring any opponents fast enough to
tripleturn, the specifics of which vary on exact timing of FF13 moves which I'm not willing to do at this time.
EDIT:
Here we go.
