Stone Giant Battle and/or Stones over Sandpoint are your two best bets - Stones over Sandpoint has both a hurling stone element and a hurling stone one-shot!
For the lich, Zombie Battle has a “mausoleum” mood, and several battle sounds that include skeleton/bones sounds for a dry, desiccated lich. The Wrymwraith set has a number of spell effects that are commonly used by liches (energy drain, finger of death, desecrate), or you could use Spellcaster: Sorcerer, Wizard Spells (Ezran) or 5e Wizard Spells, or Cleric Spells (Kyra)/5e Cleric Spells.
The Zombie Battle and Undead Battle also have some “undead flesh” sounds - add to that some roars/claw attacks from The Spider’s Web (which has owlbear sounds that sound like an eerie bear howl) or Tiger Battle (bites and claws) should suit your undead dire bears. You could also use some polar bear sounds from Good Mead as well.
Arkona Palace has a mood called “marai rakshasa battle”, from which you can cull some rakshasa roars/hisses/etc for that foe.
For your gnoll there is the Gnoll Battle, though you’ll want to turn off the extra gnolls in the fight (unless your lead gnoll has minions!). For the flooding cavern, I’d use Down Comes Rain. That has a pair of moods, “they’re breaking the dam” and “broken dam”, both of which have some gushing flood waters (though you’ll likely want to turn off some of the creaking/rain, etc.). To place it more accurately in a cavern, you may want to co-opt some elements from Flooded Cavern. While Flooded Cavern is a calm, peaceful set, it does have some drips and echoes that could be useful, along with a “wading in ankle-deep water” element that may suit. There’s also a waterfall one-shot that could be of use.
Also, the Goblin Arrows set from Lost Mines of Phandelver has a mood called “twin pools cave” which has a continuous waterfall sound in a cave, so could certainly suit for a filling cavern.
I hope that helps!