What tier level troops do you use for each zombie lair level?

I have t8 riders doing level 30 lairs with a single shooter.

It is not as simple as tier level as your march strength against lairs will depend on your hero levels, troops types, hero boosts for your troop types as well as gear, chips, glory etc. and all other boosts that affect your troop strength. It will also depend on which other players join your lair attacks.

In very vague terms T7 will be good for around lairs 15 plus or minus a few. T8 will do for around lairs 20 plus or minus a few. T9s will be good for lairs 25 and up.. YMMV of course depending on your buffs and heroes and how strong your allies are.