How do you protect your cob cannons in Survival Endless?

So I've been playing survival endless for awhile and it just doesn't seem possible to get past wave 10. The gargantuars and giga-gargantuars are just too much for my defenses to handle. I know most players use cob-cannons for damage, but the problem is if I put my cob-cannons too far back imps and diggers get to them and if I put them towards the front they get crushed or eaten. Is there a way to get them right in that "spot" where they are protected from imps, diggers, and just the zombies in general? P.S. I play on android so I only have 9 seed slots instead of 10. Website