Empowered Autogenerator in exile (Empowered Autogenerator exiled by Myr Welder).
Myr Welder and Umbral Mantle on the battlefield.
Umbral Mantle attached to Myr Welder.
Myr Welder does not have summoning sickness.
({3} magic symbol) available.
Steps
Activate Myr Welder using Empowered Autogenerator's ability by tapping it, putting a charge counter on Myr Welder and adding one mana of any color for each charge counter on Myr Welder.
Activate Myr Welder by paying ({3} magic symbol) and untapping it, giving it +2/+2 until end of turn.