So first Legion list.

Vader: Sabre Throw, Force Choke, Force Reflexes. 230 pts

Stormtroopers: +1 trooper, DLT-19 Trooper. 79 pts

Stormtroopers: +1 trooper, DLT-19 Trooper. 79 pts

Stormtroopers: +1 trooper, HH-12 Trooper, Grappling Hooks. 92 pts

74-Z Speeder Bikes: Long Range Comms. 100 pts

AT-ST: Long Range Comms, DW-3 Concussion Grenade Launcher. 220 pts

800 pts

Command Cards: Implacable, Ambush, Push, New Ways to Motivate Them, Assault, Master of Evil, Standing Orders.

At 800 points I’d say you need atleast 4 troop squads at a minimum. A full 5 might be better but that might be hard to pull off with Vader and a AT-ST