Poppy has never met a problem that a few hundred more friends could not fix. She runs her army like a marching band: loud, cheerful, perfectly in step and absolutely everywhere at once.
Poppy's foot soldiers hit 10% harder, so don't trade with them head-on. The usual answers are Artillery, Gunboat and Tank.
Poppy's ground vehicles deal 15% less damage, so even trades go your way.
Poppy can afford more foot soldiers than you: expect to be outnumbered and make every attack count.
Finish off Poppy's damaged units before the meter fills: the power heals them back up.
Every hit Poppy's units take fills their power meter, so strike hard in one turn instead of chipping away. When the meter is full, pull fragile units back.