It's a puzzle: you can use memorization, sure, but you can also use deduction. Typically there are far more enemies that belong in a given time period than those that are anachronisms. If you pick a mission in an easy time period -- say, in the ice age -- then you'll be able to eliminate most stuff that isn't robots or icicle leapers. If you pick something from anywhere other than the ice age or junkyards, you can quickly eliminate the skelebots that you find. Etc.
Once you've gotten it close, then you can use simple math to figure out the stragglers. That's the deduction part. If there are 10 lightning espers in the room, and there are 2 things left you have to kill, you know to leave the lightning espers alone. The odds of what is in there (and the fact that it tells you how much is remaining) make it so that this is a puzzle that is solvable with only partial information, rather than requiring complete knowledge of what does and does not belong in a given time period.
Further, since you're able to retry it if you fail, if you accidentally blow away an amoeba in a time period that does have amoebas (which many don't), then that's just made your next run that much easier. You don't have to do that with every enemy type, or shouldn't, but you've just picked up another valuable clue in your process of deduction.
Showing you the answer with color-coding would just make this an action challenge. It's a puzzle. More to the point, it's not even my idea, it was suggested by players and I just liked the idea. So I don't even really get to take pride of ownership, which is helpful for judging comments against the specific mission. And helps me say: if you don't like puzzles, you won't like this mission type. Just ignore it. Like if you don't like twitch combat, you won't like journey to perfection. Just ignore that one. If you don't like platforming, you won't like lava escape. Just ignore that one.
The goal isn't for every player to like every time of mission equally well, but each type of mission brings out unique things about the game. The cool thing is that you get to pick and choose what you feel like doing; and missions expire, too. So if the random generator just rolls a lot of stuff you don't like, that's cool: there's lots of side things to work on, and ultimately plenty of mission types to go around. We're adding more all the time, and aim to put them into a variety of categories and gameplay styles, all based around the core mechanics of the game.
At any rate, TLDR: if you don't like puzzles, you won't like this mission. But making it no longer a puzzle isn't the answer, since the point is that it's a puzzle. It's not a memory match game, but having clues does tend to help one in deduction.
Hope that makes sense! And as always, feel free to submit your own mission types, too. There's always the chance that there's something that you'll find awesome, that I'll think is great, and that we'll implement and someone else will come complain about. I think that's inevitable with the sort of variety that we're trying to put on offer here, but I also think that's ultimately going to be a strength of the game, especially as the amount of content continues to grow.