The maximum DR at 0% battery power is 50% at 250% ability strength (I believe rounding up lowers it to 248% technically).
Without [Power Drift] in the exilus, this build will have 254% ability strength. So, to do this build without a umbral forma, replace [Corrosive Projection] with [Steel Charge] and a fitting aura slot, and don't use an exilus mod. Alternatively, replace [Umbral Vitality] with a regular [Vitality].
Replace his 3 (Thermal Sunder) with [Rhino]'s Roar. While his 4 is technically not a damage increasing ability, it increases attack speed which does increase his DPS, multiplying with Roar's damage increase.
Roar has the same duration as his 2 and 4 so it works well in combination. Always cast the 2 first since it can be used to generate energy.