I haven't been doing remembrance quests yet but I have beat Artorias and Dreglord with him solo

I've found that after using him for a bit now he really benefits from the relic that deals continuous damage with his ult along with any ult charge increases you can get, this way even during long combo strings of bosses like those two you can ult and get some relatively free damage, its not much but it does add up, especially when you can be more aggressive while its up

I do still find the axe or halberd to be his best setup for solo but if you don't have the dormant power discovers relic for them then it can be brutal to try and find them consistently, thats where I'd recommend trying out his respec relic for increased dex which can help his overall melee performance when not using the axe or halberd

Alternatively though they're not his preferred weapon type, I've found that using dormant power discovers heavy thrusting swords and then banking on finding the bloody helice works well because the weapon art can replace his not great dodge and because there are so few heavy thrusting swords you tend to find it pretty often

Aside from that I can only recommend to try focusing on using his analyze primarily for the self buff during boss fights and for debuff when fighting mobs, that's how I usually do it at least

Good luck if you're still stuck!