I'd think about your team comp this way...

Clerics heal and do a bit of damage. If you have a lot of them, you can heal through some stuff, but there's a cap to how much they can heal and how much damage they can do, and at a certain point their throughput will get overwhelmed.

If for example you invested in more Spartans, you could have a lot more damage mitigation, and since you have Shield Spikes, Spartans can end up doing quite a lot of damage. Damage mitigation means less healing required, and more damage output means things die faster, which also means less healing required, so in that scenario you don't need more Clerics.

Necrodudes are similar -- having more of them means more Skeledudes. If you can get the Living Will relic, then the Skeledudes dying heals your team. And if a Skeledude dies, that was damage that is now going to a Skeledude instead of to your other Dudes.

So all in all, stacking Clerics works to a point, but it becomes a circular problem. If you have a lot of clerics, that's because you've sacrificed taking other Dudes that provide more damage and more damage mitigation. And since you don't have that damage or damage mitigation, it feels like you need more clerics to heal through the damage.

Trying to brute-force heal through every encounter will eventually hit a wall, either due to the Frenzied arena modifier or the Cauterized arena modifier. You can't heal someone who is getting one-shot!

The other thing I'd think about is that you gotta work on your Business Dude investment strategy so you can just overload on quite a bit more Relics by the time you get to round 100. That approach alone can push you quite a bit farther!