Activate Gwenna by tapping it, adding 1 that can only be spent to cast creature spells or activate abilities of creatures.
Cast Acererak by paying 1.
Bontu's Monument triggers, causing each opponent to lose 1 life and you to gain 1 life.
Gwenna triggers, putting a +1/+1 counter on it and untapping it.
When Acererak enters the battlefield, its first ability triggers, returning itself from the battlefield to your hand and causing you to venture into Lost Mine of Phandelver.
Repeat.
This loop turns Acererak the Archlich into an endlessly recastable engine by leveraging Bontu's Monument to completely negate his casting cost, allowing you to repeatedly trigger his dungeon-venturing entry. As you continuously cast the Lich, Gwenna, Eyes of Gaea happily helps fuel the casting while capitalizing on his substantial power to perpetually untap and grow stronger with a +1/+1 counter. Every single iteration drains your opponents through the Monument's life-loss trigger while cycling Acererak back to your hand, creating an unstoppable perpetual motion machine that endlessly repeats your dungeon ventures.