The monster pile is pretty full of things that will easily take a few turns to kill. That's intended in this game, as Eldritch Horror is much less about monster killing and a lot more about mystery solving. There are a few Old Ones where you will have to worry about the monsters with, but you generally don't have to keep killing monsters all over the place. The thing you DO need to do is close those damn portals before the doom track gets too high.
Your low-level monsters (cultists, maniacs, zombies) are easy enough to deal with one-on-one, even if you're not all powered up. It's the large ones, like Dark Young, you have to worry about. And if a Star Spawn is sitting in front of a gate you have to close, you can team up on that thing to try and take it down. Get everyone to the monster that you need to deal with and have them start to throw everything they have at it. I mean, when you look at what you're trying to deal with, you shouldn't be taking on a Star Spawn by yourself anyways. That thing's as big as a house.
And it is true that you need to get the right items to start taking some of this stuff on. That's why I like playing with people that start with weapons or as Charley Cain. With Cain, you can basically airdrop resources to people that need it the most (and this goes for things other then monster slaying). Getting people the right cards when they need it helps to clear away unwanted monsters.
Early game, you should be focused on getting at least one mystery solved and ignoring monsters for a while (even if it's the Black Goat). Once you have something solved, you have more time to deal with picking up resources. Usually have one or two people on monster stomping duty, buying up whatever items they can, while everyone else tries to manage the doom track.