You only use 1 hero to beat Campaign.

There is no team for Campaign content.

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As for Faction War Teams, I did focus on 1 team really hard.

It was my Undead Team.

I am at Stage 20 with my Undead team.

I can't seem to get past it at the moment.

I need to level a new champion to pass the Stage.

The Faction War Stages require specific set of skills in order to pass them.

The team I am using doesn't have the right skill I need so that is why it is stuck on Stage 20.

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As for your Arena team, It depends on your set up.

Most low level/beginner players use a Speed Nuking Set up.

The Speed Nuking set up they use runs 4 Rares most times.

Hey, nice topic

I haven’t focused on single faction outside of Faction Wars, but if I had to choose a team it would be dark elves or high elves. High elves have RG, apoth, Tayrel, and Marksman that I built up. 

Thanks!  

I've been thinking of using a dark elf team but I still need Spirithost to drop in order to make it work.

Definitely Undead Hordes :P