It's likely to be a Rampant Growth all by itself because any Green decks at the table will be ramping their lands ahead of you, but it gets particularly great when you combine it with cards like Gush, Thwart, Extraplanar Lens, Deprive
Its vital for any Water deck since she can make a Water Pokmon go from having no energy to an abundance of energy
White has no Shortage of ways to blink Zidane too
Sunscourge Champion is fine with Panharmonicon but worse than it looks because generally you want to cast Sunscourge Champion on Turn 3, which happens to be the turn before Panharmonicon hits the battlefield
What makes it valuable is the combination of haste with the battalion trigger
With both of these permanents on the field, all you need to do is play a land or make a saproling, and youll kick off an infinite chain of saproling-forests entering, triggering Sporemound, and creating another saproling-forest