So, this will be brief, I'll expand later... one thing to note; Poppy makes a terrifying snowball, but if you don't or your team feeds harder than you eat, you're pretty useless.

Choose poppy carefully in response to the enemy team. Your goal is to have a champion without much CC or damage that you can use your ultimate on, allowing you 10 seconds of free-reign beating on the enemy carries.

By the endgame, you will have to choose between 30% CDR and keep your spirit of the lizard elder or 20% by selling it and picking up an IE. If poppy makes it to the end-game, its almost a sure win, you will be able to kill 3 champions before your ultimate finishes.