Myr Welder and Encroaching Mycosynth on the battlefield.
Empowered Autogenerator and Duergar Mine-Captain in exile (exiled by Myr Welder).
Myr Welder does not have summoning sickness.
({1} magic symbol) (at most) available.
Steps
Activate Myr Welder using Empowered Autogenerator's ability by tapping it, putting a charge counter on Myr Welder and adding ({R} magic symbol) or ({W} magic symbol) for each charge counter on Myr Welder.
Activate Myr Welder using Duergar Mine-Captain's ability by paying ({1} magic symbol) ({RW} magic symbol) and untapping it, giving attacking creatures +1/+0 until end of turn.
Repeat.
Results
Infinite charge counters on Myr Welder.
Infinite colored mana.
Infinitely powerful attacking creatures until end of turn.