Who wins: Hard counter — you

Darius is stronger in the first levels — shove and deny farm.

Poppy escapes faster than you — take farm and towers instead of running.

Poppy deals physical damage — Death's Dance as your 2nd or 3rd item.

Poppy is a tank — build sustained damage and keep hitting; burst won't do it.

You out-trade Poppy; the only danger is their jungler — ward before pressing.

Poppy gets far more dangerous with the ultimate — dial back after level 5.