Well I've been using Grimer on a lot of my teams lately, more specfically this set:
@ Eviolite
Sticky Hold
Impish Nature
76 HP / 196 Atk / 196 Def / 36 SDef
~ Poison Jab
~ Shadow Sneak
~ Fire Punch / Ice Punch
~ Pain Split
I can never seem to replace grimer on my teams because it does so many roles. At first I thought it was one of those niche pokemon that has a "beats one pokemon all the time but sucks against everything else," mentality surrounding it, that pokemon being Mienfoo of course. With base 80 HP and Attack and base 50 Defenses, Grimer can be a really effective tank. It beats ALL fighting types 1 on 1 (Croagunk is indeed tough but Grimer will usually come out on top if Fire Punch is used) including Timburr, Mienfoo, and Scraggy, even if Scraggy carries Zen Headbutt. It also checks almost every Grass and Bug type in the tier like Foongus, and Shroomish. It can pick off Ghost types that like to switch in with Shadow Sneak, and it does a hefty 36% to Missy, which means that if it switches into Poison Jab (which it often will) and expects you to switch out and uses Nasty Plot, you will actually beat it most of the time. Many potential counters to Grimer, like Abra, are picked off with this move also. It can also finish off weakened sweepers like Dweeble, Clamperl, etc.
You might wonder why you would use Grimer over something like Shelmet, Missy, Croagunk, and even Koffing, but Grimer has advantages over all of them.
Grimer much more offensive than Koffing, Shelmet, and Groagunk and is not vulnerable to Knock Off like the others are. Grimer is also immune to Trick and that would severely cripple all three of them. Koffing and Shelmet also lack priority, which Grimer has. Missy is still vulnerable to Knock Off and will lose against Scraggy and Timburr that carry Payback most of time while Grimer can handle these pokemon more comfortably. Missy also doesn't have priority (well it does have shadow sneak technically, but who would ever use it?)
Coverage is a little bit of a problem of course. I prefer Fire Punch because you can't do anything to steel types like Magnemite, Ferroseed, and Bronzor otherwise. Ice Punch is useful for Archen, Lileep and Ground types that switch in. Another perk that Poison Jab has that is often overlooked is it's 30% chance to cause Poison. This means that walls that want to switch in on Grimer, like Porygon, Hippo, and Lileep will eventually become poisoned and their job is much harder when they are losing 12% health every turn.
Grimer is really underused and underrated, I don't even think it breaks the top 75 in usage, probably because people think it's useless because it has difficulty with ground types which are always on Sand Teams, but it actually is very good.