This time around it is a Trainer Class, NOT a specific NPC, but OH MAN do I hate them. And it's not what'd you probably THINK would be hard mooks, like you would Ace Trainers or Veterans.
It's the freaking Researchers in the Shadow Pokemon Lab / Cipher Lab in Colosseum. Those guys with primarily Magnemite / Magneton and Voltorb / Electrode. Who spam Thunder and occasionally explode.
Lemme give some background for you. It is at this point the game is transitioning from midgame to lategame, this being the penultimate dungeon before the Cipher Admin rematches in Realgam Tower one area later. You'd think by now "oh, I should have some good options to combat Electric types now, right?" The answer is NO.
You have like...three options for Ground types before this, with the only other one, Vibrava, being at the tail end of the lab with two fights after it. The other three are Quagsire, who gets Earthquake at level 42 and should be JUST getting the move around now (and is thus the best option), Gligar, who literally has no natural Ground STAB, and Piloswine, who if you use a Time Flute on gets Dig instantly. Piloswine Dig sounds good until you realize that means your teammate is taking more Thunder spam unless you opt for Protect, which is iffy for some mons because A) the move relearner doesn't exist in Colosseum, so moveslots are really valuable, B) Protect isn't really commonly good until Realgam Tower rematches where you might get some Earthquake spam off and C) that means the matches drag even if you have a Ground type. So, to recap, the game gives you ONE good option, one mediocre option, one awful option and one option after the Researcher spam ends.
You have to go through FIVE of these annoying jerks to get through here, with an extra sixth with some jokes like Electrike and Chinchou randomly thrown in without Thunder, instead packing...Shock Wave and Thundershock? And one guy randomly seems to use two Spark Electrode with Explosion too. Either way, Thunder spam may not sound all that bad with the lame accuracy, but the law of averages WILL add up and most non-resists or mons not named Altaria will be easily 2HKOed by Thunder in the low 40s.
And to top it off, the BOSS of the place uses Rain Dance and Thunder, because you aren't sick of seeing THAT move yet. Your mons are gonna be dropping like flies throughout the dungeon, and as a final screw you, you get the TM for Earthquake RIGHT AFTER YOU BEAT EIN.
It's just one move, but you'd be surprised how obnoxious it can be. And remember, there's NO HEALING MACHINE in this place, so unless you like tediously switching via the PC near the start, have fun constantly leaving to heal up unless you spam Revives.
And yes, this could fit in the "little things that annoy you thread," but honestly the Researchers are the focus and they are the WORST.
Edit:That alarm guy uses Rain Dance Thunder, and Ein literally hit every Thunder even without Rain Dance, ughhh.