It was just a basic, undead slayer with a couple decent mods. I can't seem to find the good one I had tucked away. Using consecrate and EOO I was doing around 60 damage. I'll go shopping and see if I can come up with a better bow. I'll also alter my suit a bit to get my resists up.
That's about what I do dmg-wise, and I have no problem soloing the Skelelich.
But I also use Mischief Maker, and with Divine Fury, I fire them off almost end to end, and it chops him down quick.
Its more than just damage, though, you gotta learn how to "dance" with him. Like most AI in the game, the path it pursues you is pretty plain after some observation, and you should have the timing to stop when you are due for your next fire.
With all of that, the only thing you need to worry about is that pesky summon he does, which isn't an issue outdoors (sometimes when he does it inside a tomb, you can get pinned). As long as you keep your health tip-top, this is a minor annoyance at best.
For the spawn, remember that the fiends do an area of attack when another monster is nearby... and it stacks. So when a bunch of foolios manage to bunch up some fiends on the Skelelich when fleeing.... break off your attack for a bit. Otherwise the next summon will give you an instant dirtnap. The spectral armors (and sometimes the brigands) can be ignored for a few moments, but you should lead them away or take a few minutes to deal with them. The other undead you probably could ignore all day.
Another thing I've noticed about the Skelelich is that he won't heal himself unless he is in dire straits. I whittled boney down to half life, took a break to clear out a few fiends that were hounding me, and came back like 3 minutes later to find out he didn't heal at all.
Anyhow, you'll get into the groove. Remember that high damage isn't as good as slightly lower damage when you can get in twice the shots.