Honestly, I have never a dreadful battle in any main stream pokemon games.
Pokemon Colosseum:
1st time Venus: Her attract and Suicune were extremely difficult, one of the few Pokemon trainers that I had to fight more then twice.
Pokemon XD:Gale of Darkness:
1st time Gorigan: I was young when I played this game so I did not know how to counter Protect-Earthquake. I believe it took me 10 attempts to beat him. I remember hating him with a passion. I actually had to go through this game twice because the next time I played after defeating Eldes, my save file got corrupted. The second time I fought him, it still took me about 5 tries to beat him.
Ardos: That shadow end Snorlax
3rd time Eldes: The Latios and Latias could sweep entire teams.
Pokemon Battle Revolution:
Master Battle set 8 Master Class Canata: the use of trick room and spore parasect.
Master Battle set 6 Sashay: Her pokemon were extremely bulky and difficult to overtake. This battle took me forever.
Master Battle set 7 Master Class Razma: A very difficult battle that can change at any minute.
I know what you feel about Venus, she's a b*tch :/. I surprisingly caught her Suicune very easily, however, her attract + paralysis + confusion strat is awful :(.
The first time I battled Ein, I was surprised by him, I didn't know I would encounter a boss in this particular room so I didn't heal my pokemon and my Ampharos was dead which was my best pokemon against his team. After lots of confusion/paralysis/crit hax and potions used, I managed to beat him and capture his Raikou but this was awful :/.
I'm playing XD right now and I found trainers with very original strategies that can be very effective :
- There's a dude in the tower colloseum that says that he likes fighting type pokemon. I thought : "well, that's not a problem, I have Gardevoir in my lead spot". He sent out Hariyama and Misdreavius ("WTF Misdreavius ?"), he used fake out against Gardevoir and Misdreavius used Imprison to prevent my pokemon from using psychic and aerial ace.
- There's a cypher grunt that leads with Wobbuffet and Shedinja which means that if you don't have anything that can hit Shedinja in your lead spot, you can't switch out unless you kill the Wobbuffet which is very risky. I luckily had leech seed on my Jumpluff to kill Shedinja. The funniest thing about it is that my Gardevoir traced Wonder Guard, I didn't know this ability could be traced, that's good to know :D.
- There's another trainer in the tower colosseum that leads with a Wobbuffet and a Misdreavius. His strategy is to use perish song and trap me with Wobbuffet. The funny thing about this is that my Gardevoir traced Shadow Tag so everyone on the field would die to perish song due to being trapped :D.
These trainers may not be particularly hard but at least they have very original strategies that could be deadly in some situations.