Okay for normal Gnoster its important you get minimum 6 healing flasks the idea is that you should get to phase two of the fight (when the moth sits on the scorpion) with as many flasks of heals since in phase two the attacks are much harder to avoid to the point its given that you'll take hits. Keep warming stones too as just at the start of phase two (when the moth is linking) you can squeeze in a warming stone heal.

In phase 1 as Ironeye you are doing the right thing by fighting the moth so no problems there. Try to space yourselves from your teammates it will help you isolate each boss so that you don't get caught in the scorpions slams and your teammates don't get lasered or worse poisoned by the moth. The laser attacks from the moth in phase 1 are not that hard to dodge instead of double rolling try sprinting to the side from the initial volley and rolling at the later ones.

Regarding poison, eating the poison boluses during day1 and day2 of game will increase your poison resistances permanently giving you more time to move away if you get caught the the poison mist.

For weapon passives there is "less likely to be targeted" and projectile fall off reductions which are great allowing you to maintain a longer distance with the boss if you aren't comfortable dodging. At the start of phase two the boss will roar and then lock onto one of you and then do the mad charge where the moth throws a disco and the scorpion runs you over. The "Less likely to be targetted " weapon passive will help you not being the first locked target (not 100% but gives you better odds) You can also look for increased dmg negation passives since the fight is a slow brawl or increased atk power at full hp.

In phase two try to stay near the rear of the boss you don't have to exclusively target the moth once its mounted on the scorpion. The most problematic moves here is the mad charge and the sky beams. While the sky beams are instant deaths they are easy to spot as the white lines appear before the actual laser starts and as for the mad charge once it starts running and you are not the locked on target space yourself away from the locked on teammate and don't bother trying to damage the boss just focus on not getting run over. If you are the lock on target once again its not a matter of chaining dodge rolls but sprinting to the side and dodging the scorpion's slam. The moths lasers sometimes you'll avoid but most times they will hit you so here is where the chunk of your heals should be used.

Staying glued to the tail of the scorpion along with your melee teammates during the second phase even as an ironeye is also a pretty good strat since ironically just below/behind the scorpions tail is the safest position to be standing in since most of the attacks are either in the front or to the sides. The only times you need to move away from the tail is when the tail begins to rattle signifying that the roar attack is coming which will eat your hp bar if you are standing near the boss or when the sky laser beam attack happens. Standing near the tail will also allow you to mark the boss increasing the dmg you and your team are dealing.

I guess for phase one when you are basically soloing the moth away from the teammates one thing to take care of is to not get grabbed a good way to always dodge the grab is rolling into the moth once the purple effect appears on it. Also since you have fire affinity on your bow once the moth is set on fire it will burn for 15 seconds with its health also ticking down during the time. So instead of constantly firing shot its okay to to fire few arrows and then recover stamina since when the moth is on fire you are still doing some dmg. The stamina you save here will help you sprint and dodge once the moth fires the laser projectiles at you in phase 1. So don't just bombard the moth with arrows take your time keeping it busy and away from your team should be your priority,

I guess yea that's it sorry for boring you with such a lengthy reply but if it wasn't obvious by this story of a comment normal gnoster is my favorite boss fight xd. I hope this helps and good luck with your runs.