It is near impossible to get a 5 star victory on maxed out bases!

[Ravenlord]

My Brits 5star often enough but once all towers are gone I'm happy with 3-4stars - at 2500 medals the extra star is not worth the trouble. My Romans use heavy tanks and are happily playing around 1200 medals where stars & loot are plentiful!

 

[Pharon]

If you're having a lot of trouble with maxed walls, there are two easy things you can do -- use engineers or use more ranged troops (rifles/artillery). No base is immune to 5 stars if you know what you're doing.