Single player, by 
There are two things about the designs of these levels that really stick out to me, so let’s start there.
The first is the trigger crates. There are a lot of them here, and I think some are handled better than others. The ones in the first level are both pretty bad… you have to take a long path to find each one, then take the same path again in reverse after hitting the crate, only all the enemies are gone and you’re just running around to fill time. To get to the first crate, you got bounced by some horizontal springs, but on the way back you have to avoid them… I’m not sure whether this is an intentional challenge or confusing, but I couldn’t find any other path to take.
By contrast, there’s a trigger crate in the second level that is handled much better: hitting it takes you into a new area of the level, with new enemies and other challenges, and you have to navigate all those things before you get back to the main flow. That’s actually interesting, rather than filler.
This may be my personal preference talking, but I think it’s probably better to ensure the player sees the trigger scenery blocks first, before they go hunting for the trigger crate, so they know where they have to go back to. Otherwise you get moments like the second trigger crate in the first level, where the path branches in two directions, one with the crate at the end and one with the blocks at the end, and which direction you choose at random dictates how much time you’ll waste on backtracking.
The other main thing here is that there are points that seem to expect the player already knows how to beat the level. This is a common/easy level design mistake: the person making the level obviously knows where everything is, so why doesn’t everyone else? The less problematic cases of this include unlabeled warp events, or mandatory coins (in the secret level) hidden inside walls; the more egregious case is in the third level, where you twice need to shoot some normal-looking walls (using bouncer bullets!) or else you’ll fall into a bottomless pit. (The pit doesn’t even hurt or kill you, so you have to cheat or else quit the game.) I’m comfortable saying that this is just objectively bad design. (Some of the trigger crate hunting also feels to me like a bit of mind-reading, but I recognize that some players may be more into hunting than I am, so that’s just opinion.)
Those are the main offenses of this pack. There are a few things that are just buggy—bad masking on the ends of vines or poles, springs that don’t keep your x/y speed properly—but whatever, those are easy fixes. Giving the player a whole bunch of powered-up seekers right before facing Bilsy renders the fight meaningless, but it’s never been easy to design a compelling boss battle/arena.
Apart from those focal points, though, I think these levels are decent. There’s a good mix of enemies and pickups, with various nice features like stomp/destruct blocks, springs, vines, and poles to mark that these are distinctly JJ2 levels. Trees and lakes both make appearances in places to make different parts of the levels feel distinct from each other. Sometimes there are little platforms and sometimes there are larger walls… the walls look a bit flat at times but that may be a consequence of the tileset choice. I think in general the moment-to-moment gameplay is pretty solid here, but some more thought needs to be put into determining what moments the player encounters in what order, and how the player is supposed to figure that out.
