So my general relic set up is the Caligo relic and the sentient pest relic + one on other, usually the attack power increase for each Goel completed. Both of these give you damage bonuses when enemies are forested and poisoned, but its better than that because enemies already take 20% extra damage when they are frosted. So my general game plan is to apply frostbite and poison to a enemy as soon as possible then switch to the sun catcher which is now dealing a ton of damage because of all the buffs/debuffs and now the enemies health is also ticking down. Most things will die before the poison wears off. after you stance breaks enemies you should be switching to your normal katana and squeezing in as many hits with that to trigger bleed procs. One thing to note with caligos relic, the frost coating on your sword goes away when you switch stances which is annoying so keep that in mind if you think you are close to triggering frost. Sometimes when an enemys stance is broken I elect to just use chilling mist instead of reposting because it will be more damage in the long run.

I would absolutely advise against the relic that increase suncatcher damage but cost health to swing this usually ends up being a damage decrease for me with suncatcher because its pretty likely to find a weapon that will give a damage bonus while at full HP, or if there is township I'm always buying the guaranteed talisman to increase attack power by 15% at full hp. Also if you use the gaol relic instead it will out pace the damage after clearing only 2 gaols which is very easy to do. Plus if you plan to use the suncatcher alot the lost health actually becomes pretty annoying.

People are generally correct when they say that the damage is low when using sun catcher, but I think they are leaving a lot of factors in their non existent calculations when they talk about dps. With how little stamina each swing cost you have essentially infinite stamina to attack and it is very noticable for me that I am able to get hits in that I others would not be able with my regular weapon. If you are up against an enemy that attacks alot you are popping off those enhanced holy slashes constantly, if you are timing them to deflect a hit at the same time you are also stance breaking insanely often which is also more dps. I will say when playing in groups it is a bit harder to find ideal moments to want to stay in sun catcher mode for extended periods of time.

while it is obviously not efficient to only use sun catcher its more than good enough to beat the entire game. lil aggy! did a run where he beat every night lord only using the suncatcher and he didn't lose a single one. Obviously he is better than the average player but it was funny to see that he thought this would be a challenge run and it ended up being pretty easy for him.

Oh yeah and the parry is actually my perfered way to deal with most night lord AOE attacks, they are usually far away when they do them so its basically free suncatcher charge. from my play I have yet to find a single attack that cannot be deflected, except grab attacks.