A quick, but pretty useless method considering how easy it would be to just make 10 grizzlies.
Anyway load up a custom skirmish match against easy ai.
Go into settings, give yourself 9999 resources, and place any positive skulls on.
Load up death match, whilst playing as Forge, one of the key components of getting this achievement.
Build the vehicle bay, proceed to build 10 grizzlies. You should get the achievement, Happy hunting.