Probably would be easier to give more specific examples since that is an extremely broad question. If you mean simply stopping full screen hits from putting your character in hitstun/blockstun, use super armor (which itself also varies in method) that absorbs the full screen hits. If you mean outright negating them, it depends on the full screen attack in question; if the full screen attack is a helper or just P2 extending their body (think Omega Red for the latter), use a reversaldef that sends them into custom state where it will activate destroyself if the target is a helper or a selfstate if the target is P2; if the full screen attack uses a projectile sctrl, I think you're out of luck there then.

Negating the actual full screen blast shouldn't be too hard if it is a helper with the reversaldef method I mentioned previously, although you won't be able to negate any blasts if they're projectile sctrl based (unless hitting P2 also stops it). Anyway, afaik, MUGEN handles invulnerability a lot more differently than most FGs in general (it probably also helps that commercial FGs are self-contained environments, so, they can establish compatibility far more easily) and there are very, very little ways to get around full-on invuln in MUGEN asides from the classic reversaldef way (and that only affects the thing doing the hitting, so, if a helper is doing the hitting, P2 won't be affected) or the far more complex reversaldef+proj+helper exploit that was discovered here.

The docs go over it but basically. If you have 2 projectiles. One is priority 6 the other is priority 2. The priority 6 projectile will destroy the 1st and 2nd priority 2 projectiles it makes contact with. It will be destroyed and destroy the next priority 2 projectile it hits.

Priority 6 hits 2 = now priority 4
Priority 4 hits 2 = now priority 2
Priority 2 hits 2 = both cancel.

If it was
Priority 2 hits 1 = now priority 1

Its actually a pretty neat system that would be annoying to do with helpers. Shame projectiles have so few other useful features.

It has a max setting iirc but i don't know what it is. Regardless if you set it to that value itll behqve the same. The projpriority of any projectile that hits it will reduce its value by that priority.

Priority 9999999 vs priority 9000000 will give you a priority 999999 projectile. Hit that with priority 999998 and you have a priority 1 projectile.