17, I think. I was getting spammed by zombonis and catapults as early as stage 10, which ate up my spikes something fierce. The gargantuars kicked in on 18, so I was too busy using my cob cannons on them to defend against the horde of diggers who then happily annihilated my backline.
I think what I need to do next time is to avoid having an entire back row of winter melons, using two or three of 'em to keep the enemies up front slowed, and then stuff a couple more gloom shrooms back there to defend against the little bastards.
I'm wondering how the game selects enemies and the like - I thought I'd read that gargantuars didn't turn up until 40+ flags in...
CAUTION: I reserve the right to remain crass, opinionated, cantankerous, and argumentative at all times. Expecting me to behave otherwise will lead to conflict.
Completely random on the zombie selection I believe. After flag 4, anything goes I noticed.
Two gloom-shrooms in the back will kill all miners and any bungees as well. Basically, two next to each other is a good defense against imps, miners, bungees, etc. I used two leaf plants in the back to protect against the basketball carts, the rest of all gloom-shroom love is in the middle and front.
Here's my favorite setup so far that's I've come up with: http://endless-sphere.com/screenshots/pvz_good_survival_setup.jpg
It's an expensive one, but once it's setup, all the lanes become death for just about anything. It takes a long time to advance in flags, so I have no idea if the setup will break 100 or not like my old one did.
I had a nice setup that got me up to 50, but then a Jack-in-the-Box wave took out all but two of the glooms in the pool. I managed to survive for four mroe flags after that, but I never fully recovered and my lawn got obliterated.
The only problem with your build is Jacks. They'll be a very big problem for your ground Glooms because that's where Jacks start sploding the most often. It cascades too, so if one goes off, the others have a much higher chance of chaining.
The only answer I've gotten so far is double Fume and double Gloom on the ground (third is just extra) with http://h.imagehost.org/download/0111/Elegant