Alrighty. Let's start with,
Tips
1. There is no way to use campfires/food while inside. Take care of that outside. Only potions can be used inside. Also Crisis escape does not work.
2. The orb in which begins the monster summoning spawn CAN BE USED as a campfire-like object, saving you from direct liniar attacks like Claimh Solas's charge and Master Lich's Fireball. Good placement is required.
3. If the game provides you with it, use it. Use any buff you can grab your hands on.
Now for my,
Method
1.Sneeze through giant spider, small golem, tabhartes, siren, and succubus. I recommend hanging back at the initial spot behind the orb so they dont aggro you and use a long range attack to slam a dent in them, so they will be easy pickings as they get closer. I do firebolt/fireball as I see fit for Tabhartes and up.
2. For Claimh Solas I immediately begin using movement speed boosts. March song, Fleet feet, Shylocks step, Shield of Trust etc. You don't want to get run over by him. squishy or not. His hp is fairly easy to snip off if you use Ninja/ Fighter. Lullaby and Lance smash or something stronger ought to make a nice dent for you to work off of but make sure to not use something slow or he will punish you.
3. For Merrow, the waifu (please be gentle) her attacks are fairly easy to avoid, but use movement boosts just in case. Her debuff isn't as bad as grim reapers but it may still cause a problem later on. I use firebolt/ninja for her.
4. The key element in Grim Reaper is to not get hit. It's going to be tough even with movement speed buffs since his slash ranges have been doubled in size. If you find yourself using too many pots, then your falling right into what I call the "Cave Trap", which is basically getting royally screwed by pot poisoning and fighting a steroided Glas. I use way of the gun for Grim Reaper.
5. For Master Lich I immediately use shadow cloak and get close to him and the ghast that first aggroed (Only one will target you at a time). Lullaby, Battlefield overture, and Tranz should set up the perfect Sakura Abyss, eating up 75% of his HP or 1 shot him if I crit on all hits. If I'm lucky enough to crit then It's smooth sailing, but if not, I use demigod and I start snipping off the rest of his HP with Ninja. However if I have to use tons of pots to stay alive due to his attacks then I'll be falling further into the Cave Trap, especially if I get caught with that purple bubble bs that negates the usage of pots until it's gone.
6. Glas. The Master of the Cave Trap and the key element of screwing you over. To put it bluntly, if you are pot poisoned, you will be since his shoop da whoop pretty much puts you into deadly now, no matter how much HP you have. The point of the trap is so that the combination of Grim Reaper's debuff, and Master Lich's hellbent damage (no pun intended) cause you to be pot poisoned early on, resulting in a Trap where you fight a regenerating, steroided Glas Ghaibblean with little to no stats. A fight in vain. I spent 20 minutes in an epic battle against Glas the first time I tried the Cave, with most of my stats at 0 and Glas just kept regenerating. Now if you manage to begin fighting Glas without pot poison (or not as bad) try to catch some breathing room until the Ghasts despawn. I use Doppleganger (by killing the gargs) to avoid most of Glas's lazers, or I use smokescreen. Wotg, Ninja, Fighter, and Lance smash works great, the last one being the riskier one, but if you can land those hits, Glas won't pose much of a threat.
Hope this guide helps!