I can’t rightly say what I find most personally tedious, but I can say what take the longest to farm and how repetitive their farms are! I can say what monster farm I love most, but I’ll save that for the end of this post~
The most infamously tedious by sheer kill rates alone would be Zmei and Jinn.
Zmei have absolutely no farm methods outside of clearing out their spawn points in CF, and their respawn time isn’t particularly speedy and hinders the overall rate. You’ll be doing the same thing over and over again for Zmei for a long while.
For Jinn, you have a few options (select missions, overworld spawns, or eternally Hell Eternal) and you will get some passively during scenarios as well, but the kill rates for all Jinn methods are in fact slower than Zmei… by far. Jinn will take a long time no matter how you do it, though you will have some variety.
Shades used to have a very fast farm method, but that method no longer exists so it feels a lot more tedious to people nowadays. They’re still scattered about in a number of places and missions. Luckily, they’re a bit more numerous than Jinn and they are grotesquely cute.
Komainu are tedious and, unlike other monster types, not necessarily feasible to farm solo depending on your gear. Learn to love the Kantoku Temple, because that’s the only place to find them.
Scarecrows can be tedious and slow similar to Zmei, but you can get a good chunk of them while doing scenarios. Personally, I enjoy their solo farm in the Besieged Farmlands - you can get a few Revenants while you’re at it!! I just like Revenants.
Other monster types that are oft-lamented (like the Scorpions mentioned earlier in the thread) actually have farm methods that can get 1k or more per hour. The difficulty for those usually lies in organizing a group and ensuring it stays together long enough to complete set goals for everyone involved.
Personally, I ADORE Revenants and their farm. They’re good birds!! They aren’t tedious to me and I know all their quirks. I love them.