Pokemon Battle Frontier
On Emerald, if you go up against a Blissey and you don't have a pokemon with decent physical moves, you are completely fucked. Blissey will spam Double Team, making it difficult to hit that fat *****, and will heal using Softboiled, so when you do manage to hit her, she heals. Meanwhile your pokemon are choking to death on poison. The fight will last for ages and you will never win.
On Platinum(I think), I create a team of some of the strongest pokemon that could be used yet carefully balance weaknesses and strengths. I breed them for the right natures and moves, EV train them, level them up, give them good moves.
And then I fucking lose to a Whiscash(or Dewgong) that manages to hit with Fissure/Sheer Cold/Horn Drill 3 times in a row. There's a 30% chance each one of those moves will hit and it fucking got me 3 times and destroyed my team.
And on SoulSilver, I was using Gardevoir in the Battle Hall. Not the best pokemon to use but she was doing pretty damn well. On the 99th battle, just before you fight the Frontier Brain for the second time, I ended up against a Honchkrow. I'm aware it can 1-hit-KO Gardevoir with a Night Slash but I wasn't worried as Gardevoir has Thunderbolt and good S.Att while Honchkrow has a pretty crappy defense. Plus Gardevoir, had Choice Scarf, meaning I was sure to 1-hit-KO Honchkrow before it could attack and I would get another easy win, in theory.
But no. Honchkrow had a fucking Quick Claw. Which is a luck item. It has a small chance to be randomly activated and it did. It killed Gardevoir. Fine if the Honchkrow used something that used skill like Sucker Punch or Focus Sash, but I lost due to pure luck. Fuck you pokemon, I'm not playing you again. At least not for a week or 2...
