Nice one. Though, I haven't killed Mergo's Wetnurse, I killed Erebrias, Daughter of the Cosmos. It's Erebrias I thought pretty easy at 72. As long as you can avoid her charges and wear anti-frenzy gear or have sedatives then you'll be fine. Her blood spit caught me twice on the kill attempt and nearly had me frenzy, which would have killed me. I like how batshit crazy Alfred went. His wheel had chunks of gore all over it! I resurrected her though. I need to go kill Mother Brain, then the Wetnurse, and then I think I'll do Chalices before finishing up. I have no intention of getting Platinum so I'll just reroll and have another go through the game I think, or hit NG+ depending how I feel. I ended up just gunning Erebrias down, do love the Evelyn with high Bloodtinge.
The Chalice dungeons seem to have to be done in order. I did the Pthermu (spelling?) and then Central Pthermu, and now will do Lower. Each one seems to drop off the final boss of the last, and the materials are in boxes around the levels. They don't seem to be too hard to find but have to be done in order. Erebrias drops a Chalice I can't activate and I don't have the materials for the Loran one from Amygdala either. I think I'll get the materials for them from the Lower Pthermu chalice, or the one below that.
Apart from the cool bosses, the best reason to do them is the weapons. Since I'm nearly done I've read up on the game, and the weapons there have different gem slots - which is awesome. So you can get weapons that normally don't have Waning slots and turn them into Fire or Bolt weapons, which I plan to do with a second Chikage.