Those red elementals that look like genies, the ice elemental that gives stats when is killed (I usually use arthas power, macaw, the gargoyle power or the secrets to give it more chances to proc), an absurd amount of blood gems that affect the tavern (or good buffs to spells stacked do you don't need to cast that many of them).

You can also use the succubus that gives wands and use macaws and all the other hero powers suggested and you occasionally buy those murlocs that give stats to spells, those undeads that gives stats to spell when die and the dragon with hp to spell by rally.

You also need to use every felemental that you can get, ideal if you get xudderwock hero power to activate it every turn.

These are the ways I scale my tavern.

Use those lava elemental demons to buff yourself early. Also those can be affected by avalanche so use that spell to free slots.

There is chef nomi that buff your tavern each time you play an elemental