So I just tried level 8 of PP, and it took about 10 minutes long.
I used MF, Imp Pear, Goo Pea, PL, TT. All zombies were easily defeated except for one kind, *drumroll* Roman Healer!
None of my plants I brought are made to defeat them. About 8 Healer remained, only on two lanes, separated almost equally. I keep planting PF-ing my Goo Pea, 3-4 of them in each lane using powered tiles. They hardly killed them. After several PFs, I noticed 1 was killed, so I'm optimistic, until about 8 minutes passed, 3 of them in the same lane are still standing healing each other.
Finally I just toss them out using my lovely coins (actually got a free power toss from endless, but still).
Using Goo Pea's PFE which pushes zombies back is such a bad idea since they'll get stacked together in the same tile, and if the Roman Healer heals based on the tile then I think 3-4 of them in the same tile = near-immortal.
I think the ones that I managed to kill were separated by tiles since I planted Goo Peas in-between them, and before I PF-ed them.