Good enough to still use as long as it hits more then 1 target at a time. Best to not worry about it. Just make sure that you are not damage canceled when it connects for 2-3 times then finish off with chip damage.

Examples are assuming you are the only one attacking and only trying to hit with the special attack.

Devil takes 1/4 of remaining hp

Enemy has 1000 hp.
1st connect = 750 hp.
2nd connect = 563 hp.
3rd connect = 423 hp.
Chip damage to kill.

Demons takes 1/2 of reaming hp.

Enemy has 1000 hp.
1st connect = 500 hp.
2nd connect = 250 hp.
3rd connect = 125 hp.
Chip damage to kill.

Neither special suffers a multi target weapon reduction or activation lessing effect.

Making good use of the demons special family has more to do with timing your attacks based on what your team is doing and knowing when certain vulnerable key frames happen to the enemies when the special attack is more likely to connect on the enemies such as when they are in stun animations or frozen, and when others in your party can not attack.

The key frame thing can not really be explained as it differers based on your own lag but it is something you will have to learn for yourself over time through playing, and getting better at using the special.

It is also better to have it on a ranged multi hit weapon such as a slicer or shotgun because then you will have some distance between yourself and the enemy to work with. As landing a hit with demons or devils does not cause push back to the enemy as a connecting hard attack would.

Chip damage would be from throwing in a hard attack or normal attack in while trying to connect with your special attack. Causing just enough knock back to the enemy to keep them at distance while you keep trying to connect with your special attacks. The chip damage + the damage caused by the special attacks can have your enemies down to low hp pretty quickly. But you do want to try to hit with the special attack before you start doing chip damage to maximize your damage output. As the special takes from remaining hp, not from enemies max hp.

May be worth noting that "key" or "instant" frames are generally during attack startup or when enemy AI hasn't activated yet but the monster is on the field (IE Rabarta on first frame when the monster spawns will freeze them). It's also worth noting that spawns are generally asynchronous due to connection issues, and that exacerbates any issues with hitting the correct frame (this problem disappears on offline, single player capable versions of the game).

Also, yes, Demons is a very good special.

I'm like 95% sure you have tested it wrong. I'd try my hand at helping you out, but I'm sure someone will post with pretty extensive Demon's knowledge.

By default 303SNR-5 has dark as it's special attack.

Unless tofuman or Rupikachu want to tell you that they changed it from how it is in the vanilla tethealla files that is what it has.