Crumbling Farum is a million times easier than Haligtree and even Mountaintop of the Giants IMO. The only really bad section is when there's scads of those skeleton guys cause they're super beefy and will poise right through your attacks if you try to hit them while they're attacking. Good time to use moonveil and pull carefully - especially when you first go up the elevator and there are a few of them that aggro together if you walk forward that look dead unless you hit them with a ranged attack. Oh, the sentinel miniboss is also a huge asshole, but that's kind of to be expected. If you don't feel like beating him fair and square you can always run up to maliketh's door, save and quit, and then creep up behind him and kill him with poison mist without any threat to yourself.
The other section(s) of crumbling farum that suck are a few specific enemies - especially the full health sleeping dragon in middle of the level and the INCREDIBLY beefy crucible knight. I learned to just never fight either, since neither one gives you any drops, and they have a pathetic amount of runes for how difficult they are. The banished knights that hang out around the godskin duo are really nasty too but you can take them out as long as you sneak up and hit them with a few super powerful attacks before they can retaliate.
Crumbling is just dumb because well, it's crumbling and I seem to hit every shit angle on the collider geometry and slide off some part of platforming, so I've biffed a couple runs worth of runes just because of that. I already took down the first dragon almost solo, but I said fuck it and summoned Mimic just to melt thru him between the two of us.
I remember that Sentinel fight very well, shit fucking stonewalled me for so long on my strength build, no idea why either cause he's not that difficult; but maybe it's because there's no summons for it.