({1} magic symbol) ({G} magic symbol) at most available.
Steps
Activate
Marvin using
Selvala's ability by paying ({G} magic symbol) and tapping it, adding ({G} magic symbol) plus an amount of colored mana equal to the greatest power among creatures you control minus 1.
Activate
Marvin using
Farmstead Gleaner's ability by paying ({2} magic symbol) and untapping it, putting a +1/+1 counter on
Marvin.
Repeat.
Notes
The mana cost can be reduced to ({G} magic symbol) if you control a creature with power 3 or greater as you start the combo.