Simple build to use the sentience of Excal Umbra to mow down enemies. Find a small area to stand on to prevent him from walking off (eihter a box or ledge), go to Operator and watch the fireworks. Give him a Kohm for best effect.
Narrow Minded keeps Umbra from using abilities most of the time due to the reduced range, Natural Talent and Speed Drift speed the whole thing up in case he does use an ability.
Umbral mods and Adaptation to keep him alive longer (you can simply "revive" him by transferring back in and out again).
Shield Disruption and Coaction Drift can completely remove shields if your team uses those mods aswell.
Arcane Tempo to increase DPS on the Kohm
Last slot can be whatever you want but will probably require you to use another forma.