These are the main enemies of the game. Robots and Zone Dogs don’t count, pretty much everything else does (note, not in the Seeds of Evil DLC, but in the main game). 40 isn’t too big of a number, you’ll have picked off about this amount early in the game. Probably before you acquire your fourth companion.
Some particularly dangerous Ghouls are:
Pyros (throw Molotovs, sets your team on fire, note, Smoke Grenades put out fire and there are two fire immunity armors available fairly early game).
“Brain Dudes” (sorry, forget the name, the first one you meet is when you’ll hear a voice in your mind). These can Mind Control and cast Chain Lightning. Stun or otherwise burn these guys down fast.
Hunter Ghouls are ranged and deal massive damage on higher difficulties, take cover and don’t neglect getting the high ground defensive bonuses.
Tanks, you’ll probably get this before you have to kill any tanks. Tanks are Charge immune and can Hog Rush on a very short cool down. Make sure you have elevation on Tanks and root them if you can. Note their weapons can destroy cover and destructible high ground, but has a short range before it loses accuracy. There are multiple armors that grant Charge Immune to your team, learn to bait the Tank into charging a character wearing it, they’re super dumb and will do it multiple times. Due to their high armor and hit points you will often want to deal with Tanks last so long as you can keep them from being able to Hog Rush or gain any accuracy on you.