When Bumi enters, earthbend 1. (Target land you control becomes a 0/0 creature with haste that's still a land. Put a +1/+1 counter on it. When it dies or is exiled, return it to the battlefield tapped.)
Whenever Bumi attacks, put two +1/+1 counters on each land creature you control.
Turn your lands into a swarm of giant attackers using earthbend, then swing with Bumi to dump two +1/+1 counters onto every land creature you control each combat. You ramp aggressively, animate lands en masse, and crash in for huge, recurring damage—lands that die simply return tapped, so your board is hard to grind down.
Lands that die or get exiled return to the battlefield, making your attackers extremely resilient to removal
Snowballs fast: each Bumi attack adds counters to your whole land-creature army at once
Leans on lands you'd play anyway, so animating them is essentially free board development
Mono-green gives easy ramp, counter synergies, and big trample/overrun finishers
Wrath effects and especially exile/'destroy all lands' (Armageddon-style) can be devastating since your board is your mana base
Needs Bumi attacking to scale, so commander removal slows the whole plan
Land creatures still have summoning-sickness-like vulnerability and can be chump-blocked without trample
Mono-green struggles with interaction—little to stop combo or protect against board wipes besides green's limited toolkit