I write reviews on retro games and this just sounds so familiar. Could it be 'Time Killers'? Not exactly a sewers setting, but it's old, has monsters and meets the isometric shooter criteria. It's a real long shot, I know, but just throwing it out there in case it helps.

Sounds a bit like Gauntlet, but that's dungeons, not sewers. But still monsters and isometric view. Just chucking it out there, old man memory isn’t what it used to be.

Finally, I found someone mentioning this game. It's been bugging me too for a while but I finally figured it out. It might be 'Bio-ship Paladin' or 'Uchuu Senkan Gomora' as it was called in Japan. It's kind of a forgotten gem from the early 90s. It does have top-down shooting and one of the stages has a distinctly sewer-like feel to it. Plus, lots of giant monsters.

Hmm... it's a long shot but it could be 'The Chaos Engine' maybe? The monsters aren’t exactly 'sewer' types, but the game is kinda isometric and co-op, quite famous arcade shooter back then. Might be what you're looking for.

lol! You’ve got some serious memory problems dude! That game doesn’t exist, I’ve cracked every arcade cabinet from the 90s and this ain’t one of them! Good luck hunting the Easter Bunny next. 🧟‍♂️🐇

This really could be a range of games given the limited description, but one that immediately comes to mind is 'Zombies Ate My Neighbors'. It's a top-down shooter where you go around various environments, including a sort of sewer level to rescue people from various types of monsters. There's also 'Mutant Storm Empire' that has some levels kind of like a sewer and you fight against mutants.

Yo, that sounds a bit like Alien Syndrome. You're fighting off hordes of monsters and isn't exactly in a sewer, but definitely has that dark and dank atmosphere. Worth a shot?