Leyline Immersion + Hullbreaker Horror + Instill Energy

Prerequisites

Leyline Immersion attached to any legendary creature you control without summoning sickness.

Steps

1. Activate the legendary creature with Leyline Immersion attached by tapping it, adding {3}{G}{G} that can only be spent to cast spells.
2. Cast Instill Energy by paying {G}, targeting the legendary creature.
3. Hullbreaker Horror triggers, returning Leyline Immersion to your hand from the battlefield.
4. Resolve Instill Energy, attaching it to the legendary creature.
5. Activate Instill Energy by paying {0}, untapping the legendary creature.
6. Cast Leyline Immersion by paying {3}{G}, targeting the legendary creature.
7. Hullbreaker Horror triggers, returning Instill Energy to your hand from the battlefield.
8. Resolve Leyline Immersion, attaching it to the legendary creature.
9. Repeat.