The gems can take quite a bit of damage before they poof, and Garnet's future sight will help greatly, and Steven can still use his bubble to lessen damage from being thrown around
Sans can see into souls, meaning he'll be able to see their alignment and more than likely how they fight, and we see Pearl poofed simply by being impaled by a holographic sword and Garnet poofed simply by a device. And Steven's bubble won't help if he's being thrown around inside it.
Pearl definitely does not kill Undyne, and it's not blitzing if there's a long fight to be had. Plus, Sans could possibly defuse Garnet via soul control, since her souls would be her gems and she could be pulled apart into her two components. Either way, however, I think the most likely scenario is that Steven and Papyrus try and stop the fight.
Sorry for the blitzing thing, don't really use the word much in debates like these and I sometimes forget what it is used for. Anyways, doesn't Pearl have a lot more experience than Undyne, and Garnet doesn't have a soul. What I heard (or thought), The crystal gems are just gems with a hologrammatic body. Though, if Steven is aged just like the SU Movie/Future, he might have to fight papyrus.
But Papyrus is also quite an understanding and caring person. A bit of talking and he and Steven would have worked things out. Furthermore, a soul is a sort of metaphysical construct. Since the gems themselves seem to act as the source of the gem's being, that would likely act as their soul. Even then, Garnet is definitely strong but I think Sans has a slight speed advantage, he would probably be able to dodge long enough to at least let Papyrus and Steven diffuse the situation. Pearl may have more experience, but she has still been shown to be easily poofed, with a single strike from her own hologram being enough to put her down, so a flurry of spears could likely overwhelm her. Sans' soul manipulation could also bypass Steven's shield in the case of this being an actual fight.
Garnet has been able to deflect lightning before. Pearl defeated Sugilite, so that could give her an advantage. Now that I think about it, Steven might actually work with papyrus.
Pearl didn't defeat Sugilite via actual strength, though. She used her own strength against her by forcing her to give chase and then blasting the cliff she was on, leading to her own flail beating her. With Undyne, none of those things would really work. Undyne might give chase, but she'd be able to tank the blow and doesn't use giant, physical weaponry that would hurt her in the fall. Furthermore, Garnet's deflecting of lightning can be chalked up to her energy manipulation seen in various instances rather than it being a physical feat, while lightning is not within the move set of any of the present fighters. Despite all of this, I'm still more in favour of the pacifist route with Papyrus and Steven stopping the battle.
Garnet blitzes Sans after a long fight, Pearl kills Undyne, and poor papyrus.