Start a 1v1 on Barrens, standard. set the AI to Easy and the leader as Cutter. Set yourself as the Arbiter. Make your leader temple and a few supply pads and go take over another base. At this base, build turrets, shield generator and supply pads. Upgrade your arbiter, your supply pads, turrets and both ages. Buy the followers upgrade. Lock your base up and send your ghost to be destroyed. Now build 50 Suicide Grunts. When they all have finished building, send your Arbiter off somewhere and set your global rally point in the Gravity Lift, unlock your base and watch as the Grunts run for your achievement.
Note: Some people are saying that sending one unit through 50 times unlocks this achievement, when i tried this, it never worked and i had to use 50 separate units.
Start a 1v1 on Barrens, standard mode, both set yourself as the Arbiter. Go around taking the bases and building supply pads until you have enough resources to build both ages and followers upgrades. Then lock your base up and build 50 Suicide Grunts, send your Arbiter off somewhere. When all the grunts have finished building, unlock your base, hot drop them through the Gravity Lift to your leader and the achievement is yours.