Depends on the level of your druid.
If you're 70th, you shouldn't have any issue at all, boosting a pair of toons to 60th... and 35 will be rather easy, as Scarlet Monastery will likely be the hardest zone.

Moonfire is instant cast and that alone should kill lower level creatures.
You also have Hurricane or Starfall for AoE.

An alternative is to dual spec your Druid to feral.
When you're questing, a cat will kill mobs very quickly.
If you're boosting in an instance, a bear is a tank so you can take a lot of punishment.
And you can Swipe which is an AoE attack to kill a lot of mobs at once.

Cheaper auction house greens with Stamina and Agility would be good gear for a booster.