Twilight Prophet Twilight Prophet requires you to ascend and gain the citys blessing to partake in the lifegain, but its far easier than it sounds, especially if your deck creates tokens or puts extra lands into play
She's also a great excuse to play a favorite gigantic creature for free that normally might not make sense to waste mana on
Id personally trade an original version of Watery Grave (from Ravnica: City of Guilds) for six copies of the newer one (from Guilds of Ravnica or Gatecrash) but thats because I have an ever-increasing supply of decks
Lorcana Rise of the Floodborn emerald cards Bucky, Squirrel Squeak Tutor Prince John, Greediest Of All Belle, Hidden Archer Lucifer, Cunning Cat Pete, Bad Guy Dr
Most cards with perpetually gets scare me a little, if Im being honest
Once you have threshold, you can start making 8/8 tokens, and you can engineer this situation quickly with fetch lands or self-mill