As long as your buffs are 100% always up, you're generally pretty dang survivable.
There's also the fact that Greater Enduring makes up for not having Abundant Spellcasting since you can cast spells that last all day, then take an 8 hour nap and have a full spell book ready ontop of a full spellbook worth of buffs. So if you really, really want to go through the effort of re-arranging your spellbooks every rest its arguably optimal.
It's a shame that basic Enduring Spells is almost completely useless though.
I mentioned missing it on the Mephistopholes fight but I think I simply went to the wrong location. Random encounters can be otherwise avoided with Stealth.
Most of the difficult "random" encounters are forced story ones. Actual random encounters are like 4 trivial mooks. They can't really post a threat to you unless you're running around with exhaustion and low HP, which you wouldn't do if you were doing hardcore.