The phrase "bullet sponges" refers to enemies in video games that can absorb a lot of damage from bullets without dying easily.
Sentences with «bullet sponges»
Again, there wasn't much development with these characters so they felt more like typical bullet sponge enemies instead of people I hated. (thegamebolt.com)
Most missions will consist of going from one point to another, battling wave after wave of enemies or bullet sponge bosses. (thenerdstash.com)
It means again, that the levels are badly paced as you can get to a boss in 5 minutes, but then spend 5 - 10 minutes shooting a boring bullet sponge. (gamesrevisited.com)