Cael requires you to be able to clear out his cherubs, handle the tanky guys alongside him, and tank the damage+corruption he and his units throw at you. So in other words, some combination of:

Targeted damage (spells, abilities)
High power sweep, trample, and/or multi-strike, ideally with Quick, but not always necessary.
Mute
Sap
Damage-increasing debuffs/buffs
Endless/Regen/Armor/Shifting for Corruption

Every clan has some combination of these tools. Sometimes, yes, you're just unlucky and that sucks. However, more often than not, these factors are under your control while building your deck. Cael's always going to show up, so you always know he's coming and can prepare for him. Consider Cael a check to see if your deck has these things; if you don't, he'll stomp you. If you do, he's a breeze.

Here's some examples:

Banished has high armor, shifting, melee weakness, damage shields, and great Multi-strike in Fel. Advance the Cherubs to target them first, then wreck the rest.

Pyreborne have their signature Pyregel debuff for extra damage, avarice for extra gold so their units and spells are better, many abilities that apply pyregel, do damage, or both, excellent wide damage and targeted (flicker, pyreclasm, golden touch, inferno room), and even pair nicely with the Banished.

Don't be afraid to take early trials to get extra gold so you can upgrade your units and spells more, as that usually makes the biggest difference. Monster Train is a game all about Multiplying your Outputs. Seek multipliers wherever possible and you'll have great success.

Hope this helps!

no, its perfectly doable. what are your runs looking like? at cov 0 youre likely missing out on some core mechanics that makes the lategame doable.

One very important thing about Cael is to not try to fight him with a top floor only setup.
Even second floor is dangerous, top floor is suicidal, except maybe Holy Feed
You want some way to get rid of his Cherubs floor one, otherwise his waves are about as bad as normal units before with some exceptions. If you can kill the whole wave floor one that's ideal, if you can kill Cherubs without killing the frontliners go for it.
His own damage is also quite high, but he only does one hit so on floors without enemies it can be negated by placing some weak unit in front of others, like a whelp.

You can easily beat him at any Cov with any setup that isn't completely wrong.

An easier win against Cael is by having Ascend spells to use on the cherubs to prevent them from applying the buff to anyone, or Mute or Zone of Silence (the best card IMO)

These are MT101 core techniques that are good against anything and everything, not just Cael. Good old universal CC&S - partly what the titan fight was designed to challenge, I assume.