Honestly, having earned most of the achievements from just playing it on normal here and there when I can, the game definitely is far from impossible once you have a few strategies down, especially keeping in mind on most maps what doors/access points spawn what enemies, and what rounds certain enemies will begin to spawn on. By far, the three easiest so far in my opinion are probably the Shack being most obvious, then FER as an easy second, all you really need to get to round 20 is the uzi and once you get to it past the doors, of if you're lucky on the gun vendor, a sawed off or tenelli, and finally either the Space Station or the Jungle. Both of these are somewhat dependent on how well you perform early and have the dodge timing/map down. Once you get a decent sub machinegun, or even a shotgun like the sawed off you pretty much dominate the jungle by just standing on the higher platforms and earning free headshots as enemies come up, and blasting them away with the shotgun. The space station is pretty similar, and is almost trivial once you get to the final door as long as you have a decent weapon, its just tougher than FER in my opinion due to how narrow the one corridor is, and the fact there is no verticality allowing you to escape more easily from enemies.
That said, all of the stages are pretty trivial, though somewhat time consuming if you abuse the s key/submit key, and use the auto escape to almost immediately escape. This not only allows you to take no damage, but also offers a couple seconds of invul upon escape, allowing you to reposition/reload. This and, if you ever start to run low on strength keeping topped out on buffout, should easily last you long enough till you get a solid gun on any stage, perhaps even two and a dose of psycho to go with it. Easily enough to get you to wave 20, which is all thats intended as the game more or less just continues with waves infinitely till you quit or lose. The only honest two achieves I see being a pain to manage is the 15 hypnosis' from the Sqoids, since I don't believe you can just let them grab you and escape for those to count unlike every other enemy, and the avoiding traps till wave 15 which, while certainly possible, I can already see being the cause of plenty of heartache. The pregnancy one on Cave requiring 20 pregnancies without mindbreak will also be a slog, as you basically are required to pop anaphrodisiac for both it and the sqoid one on nearly every animation to make it work I imagine. Regardless, since there isn't really much to see just yet by finishing the game, I don't see it being anything all too worth getting other than just to say you have, or for the cosmetics tied to them,
Still loving the game as I boot it up when I'm feeling the itch to do so every here and there, definitely looking forward to the new stage whenever it comes around.