What I used to do to reach level 8 was to make them drink the energetic drink and keep putting them on place for them to work.

Depending on the energy they already have, once they drink the energetic they'll be hyper and do things real quickly - and the working goes faster too.

It's a tyrannic thing, I know, making them work like that, but I had no choice as I wanted to get the trophies and needed money for the upgrades.