Devoted Druid does not have a -1/-1 counter on it and does not have summoning sickness.
"Treefolk" chosen with Rukarumel.
Steps
Activate Devoted Druid's first ability by tapping it, adding ({G} magic symbol) .
Activate Devoted Druid's second ability by putting a -1/-1 counter on it, untapping it.
Activate Devoted Druid's first ability by tapping it, adding ({G} magic symbol) .
Activate Sliver Queen by paying ({1} magic symbol) ({G} magic symbol) , creating a 1/1 Sliver creature token.
When the Sliver enters the battlefield as a Treefolk due to Rukarumel, Quickbeam triggers, giving Devoted Druid and up to one other creature +2/+2 until end of turn.
Activate Devoted Druid's second ability by putting a -1/-1 counter on it, untapping it.
Repeat.
Results
Infinite creature tokens.
Infinite ETB.
Infinitely large creatures until end of turn except Devoted Druid.