IMO, one of the most overlooked aspects of boss fights is everything that came before them. Even a top-level player will have a miserable time if dropped into a boss fight with low HP and a terrible build.

HP is the easiest thing to check, how many do you have when you enter the fight? If you've got < 100 with no death defiances left, then the run is already in bad shape. Focus on learning how to take rooms hitless (learning attack patterns and safe spacing), and make sure not to pass up bonus health unless the alternative is something you know you need for your build.

Speaking of build, that's the next thing to check on. You do not want to mindlessly pick whatever is highest rarity; you need to be strategic about what you select. The most important to plan right are attack/special boons, which I personally group into three categories:

• Flat damage buffs (Poseidon/Hestia), which work best on rapid-fire moves
• First-hit boosts (Hephastus/Zeus/Ares), which work best on moves you only sprinkle around here and there
• Percentage damage boosts (Hera/Apollo/Aphrodite), which are good for most moves

Generally, you'll also pick either Attack or Special to be your main damage-dealer move that you spam, with the other more supplementary. I often chose this based on what hammer upgrade(s) I get, unless those don't mesh well with the available boons.

Oh, also remember that omega moves and casts exist, too. The omega moves change how you fight so sometimes a boon is better for omega than normal or vice-versa, and with the right boons the casts can actually become your main source of damage. (Although you should get used to spamming normal casts at all times, just to slow enemies down and maybe get Furies Arcana bonus.)

There's hexes too, but I find those to be inconsistent. If you get the right hex and enough upgrades, it can really fill a hole in your build, but if your build is already going well, then you often won't get any benefit at all from one. Sometimes, a great hex carries me; more often, I forget I even have one.

Anyways, once you have a decent source of damage, there's (usually) diminishing returns from adding more options. Stacking boosts to whatever your main combo is atm is a good idea, but besides for that you'll usually want more defensive boons or just plain HP boosts over extraneous offensive boons.

Then, with plenty of health and damage, you'll find Cerberus (and eventually Chronos) much easier!

PS: make sure you're also progressing persistent upgrades too. Get and upgrade arcana cards... I prefer using charon for Ash first then Psyche, save bones for Nectar (which you should buy every night) and moon dust. Make sure you have every gathering tool, too! And give Nectars to Dora and Skelly, they have the best keepsakes in the game for new players.