Aircraft Evolution (UP)

Once you get a spare 300 credits or so just buy the purple bomb from the shop when eligible and then drop it on whatever enemy building or unit is most deserving of it on whatever Level you're currently playing. You can also wait until you reach a Bonus Level (one per Era) and get one or even several for free.

As an aside, this bomb isn't worth purchasing. For less credits you can buy a slew of repair kits and a few super bombs.