This is how I did it.
loadout: mortal, MP-433 pistol, exp ammo, armor, sentry, grenade turret
spent points in this order
2 pistol
4 ammo
2 sentry (1st barrier)
5 sentry
2 armor (2nd barrier)
2 armor
5 pistol
whatever's left went to grenade turret but I never used it.