Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Straight to the Point
This build is completely broken and currently highest possible damage output for poppy in game. Since the Poppy re-work is said to come out in the next two months and since she is a low played champion i'm sure Riot is aware but simply do not care.
This build gives you a ton of AD, 40% CDR cap, 90% crit, 95% increased crit damage (thanks runes), 65% attack speed, 10% lifesteal, 20% spell vamp With essence reaver passive you never run out of mana, with keystone masteries + gunblade you heal like crazy.
End game your Autos and Q's will be guaranteed crits. Guaranteed autos for 1400+, Q's 2000+ every 3-4 seconds.
Simply try it. You'll never turn back.