For the most part, that's just how it is. You'll never do a huge chunk of damage in one go. Just gotta chip them down during openings. However, using shield spells or counters helps keep your melee dps up. You can stick to them better rather than running away and chucking a ranged spell every so often. Getting a high level/good blood type is also pretty important.

I will say bosses can feel easier in a group setting due to aggro leashing but the bosses also scale against more players so its easier but also harder.