Okay, breaking down Nina/Snow further:
Atk-Up Nina does 60% average, i.e. 24% damage. It's probably a bit higher with Luck-Up, but this isn't tested so I'll ignore it. With Ag-Up and Idle, there's a speed difference. I'll go with Snowfire's suggestion that it's 10% (personally, I see each effect as worth 10%, so in total it's a 23% speed spread factoring in that Nina is +1 from average to start).
Mediguard restores 20% max HP per use. Snow has 2.23 durability in Sentinel, or 1.90 if you factor Sentinel into the durability averages (I don't fully, since they don't spend all their time there. Not sure what I do). So he regens effectively 38-44% of an average bar of HP each turn. Of course Mediguard is slow as anything so anyone easily gets a second turn in (sometimes a third, probably. I'll ignore this), though that one would probably be against the defensive stance itself so does only 35% of the original damage. So combining her two attacks and baking in the 10% speed boost, Nina does 39% every Snow round, which almost outpaces the regen with no help from status whatsoever. I want to emphasise this, because I think people sometimes kneejerk overrate Mediguard. Nina the first comes close to overcoming it.
So this is where Hold comes in. I should note at this point that if you assign almost ANY stat penalty, Snow's going to lose this. The strongest FF13 accessories are defensive, unequipping one causes you to take 18% more damage. And one accessory will NOT be good enough. 45% hold resistance makes the move easily worthwhile for Nina, since it lasts three turns on average and she'll only need two to make it land. At the very least a second accessory will be required, at which point there are two stat hits. Even if you take the lowest assumptions I've listed so far (only 10% speed, Sentinel defensive not held against average at all), if you see each stat hit as being even +7% to durability, Snow loses (again, the actual number is +18%, so even if you half this to reflect "well half the time you'd equip for offence instead, I hit that too", not good enough).
Of course, even with two accessories, Nina likely still can use Hold! That's 70% resistance, which means the move is a push as to whether it's "worth" the time. But Nina can wait for a lucky string. Remember, she's 7HKOing Snow at base. All she needs is to hit Idle, have it last at least 4 turns, then hit Idle again immediately, and have it last the same duration. There's about a 5% chance of this every time she lands the first Idle. Not terribly likely, but in a long match it's going to happen, and that's just one path to victory... she could also land three shorter ones in a row, or let Snow sneak in an extra turn to start regenning (one miss only lets Snow get half his normal regen in, though, remember, which is only about 10% of his health), but have the Hold chain lasts a bit longer otherwise, etc. This is without even considering stat penalties to durability!
With three accessories it's rougher. I guess IF you let daze immunity cover Hold AND you don't assign any stat penalty for statusblockers AND you don't see the speed gap after the two buffs as terribly wide AND you don't factor in sentinel into the durability curve, Snow's got this.
And all of this is assuming Nina can't just tank through a summon, anyway. (I assume she probably can't, but I've never looked at them closely in the DL, and she's decently equipped to do so if anyone can, between ice halving and immunity-ignoring blind.)
