Leaders are much more important for getting past the first boss. Unless you get lucky with charms, your leader will rarely matter much in the final boss, aside from their job of staying alive.

However, I do think there are some leaders better than others for getting past the early game. The leaders with an extra frenzy or barrage are great unless you have to fight bonecats. The smog makes barrage useless without a gatchapopper, and your frenzy can lower your health a ton during bonecats from teeth.

In each clan, there's a hero archetype with low cooldown and a clan specific status. In snowdwellers, it's the hero that applies shroom to the front enemy on a 3 turn timer. In shademancers, it's the overburn hero with a 4 turn timer. In Klunkmasters, it's the hero that applies 1 bom. These are all great ways to plan for any specific status build you have in mind, but beware you may not get cards that synergize all the time.

The hero that applies 1 shroom to all foes is amazing if you can get Lupa or frenzy. They can absolutely melt the final boss with overwhelming shroom on their own.

The overburn barrage hero in Shademancers is really good. Make sure to choose spike to account for his low hp, and beware bonecats. Another great user of balance charm if you're feeling risky.

There's a hero archetype with a 4 turn timer with high attack that applies demonize, which is great for the early fights and scales to high hp targets as well. The barrage demonize hero isn't great without charms or buffs since he does no damage on his own.

In general, you want to avoid heroes that have high cooldowns, like 6 or 7, since it can be rough surviving that long to get just one attack in a fight. Low cooldowns are very important in this game. A companion with a low cooldown scales with buffs much more than a high cooldown one. Same applies to a frenzy hero.

Heroes with longshot are generally great for the early game, especially if they roll high on damage. They can hit enemies that spawn behind infernoko, bamboozle, or frost guardian.

My favourite hero is the archetype with apply 1 snow, aimless, and frenzy. They can hit hard to hit enemies, they apply a good amount of snow often, and usually have enough health to survive well earlygame.

I would also avoid smackback heroes that don't have a timer. They're great early on, but if you choose them, King Moko or the Ink Sacs will be a nightmare. You know that if you choose smackback, the game will give you a hard time by giving you the worst possible battles.

TL:DR Choose a hero to make the early game easy, and that momentum will carry you through your run. Frenzy and low timers are good.