Achievement: Retro-Futuristic Warfare?

[Phi]

I feel dumb. Literally beat the game, have every achievement but the one named above... how do? Thanks!

[Droopy The Dog]

Gotta have the "Future To The Back" and "Modern Warfare" talents unlocked at the same time. Then build a rifle gnorp and upgrade him with grenade launcher to start launching rocket arrows. I think that's all it took.

[Wolfling]

I also had to select 'Modern Warfare' as the arrow type in the gun shop