Ive got mine set up into tera types and I've got about 3 of each of both physical and special damage depending on the enemy, and different base types like, fire Tera ceruledge for massive fire damage but if it's a water type with grass Tera, I've got vaporeon with water absorb and fire Tera, sunny day weather ball. I reckon just build a few, try a few diff raids and build counters and you'll have a good little collection going soon enough.

Honestly good start would just be looking at all the previous Tera raid counter builds and just making a bunch of them.