Now six mana is expensive and you have to wait for the next upkeep to trigger it but switching life totals with the opponent while we have 0 or less is about as game winning as it gets
This will both accelerate and fix the player's mana, allowing them to cast huge, fancy spells much sooner than expected
Just one more than the two that hit earlier with three at six
One other subtle strength here is the degree of control you can have over exactly when cards are milled, and how many (assuming you have painlands or other unrestricted self-damage sources)
Cultivate always has a place in green decks, and heres the list of other ramp cards in the deck, to get more card draw, forests, mana, or creatures onto the battlefield: This ought to get the deck moving out of the gate
It can always come back to the battlefield so long as you have the six mana to pay for the ability, and can be done at instant speed, letting you use any unused mana to bring it back so you have it on your battlefield before your turn starts