Tigrans are pretty versatile, with well-rounded units.

I consider firecats fairly useless. Hunters are great for early game exploration.

I prefer Cat Masters to Prowlers unless you are playing against humans. Prowlers have concealment, which is ineffective against the AI, but can really mess with humans through guerilla warfare.

Manticores are your go-to unit midgame. Just get a stack and fly around, immune to ground units and wreck havok.

I don't really like Beholders. They are slow and doom gaze can only hit one unit at a time. Sphinxs are great because they have dominate.

As for spheres, earth kinda sucks because you get slow summons and you can't stone skin your manticores. Death strike helps to make up for the fact that the Manticores don't have any heavy physical damage hitters.

Basically: Have some spells from the Death sphere. All right. And would I need spells from the Life sphere to counter-balance?