While it wasn't specifically me, friends at school at the time apparently had a hard time with Ghetsis in BW. Apparently his Cohagrigus was too defensive and got away with double protect pretty often and their teams went down to Toxic.
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+1 for Ghetsis, but i'll also chip in the double battle with Mars and Jupiter on Spear Pillar. as a 10 year old i had a poor choice of pokemon for my team and with my boi Barry literally sending out a Munchlax versus a Purugly and Skuntank, i knew i was in trouble.
First time I played Black I had a hard time battling Ghetsis and it was his Hydreigon that was hard for me to beat. I also struggled against Giovanni on Yellow since he was using ground type and I was using Pikachu who was my strongest Pokemon.
ghetsis, there is no competition lol like i've had points where other team leaders are difficult (maxie/archie when i was a kid, i had trouble w cyrus in pearl a while back, i remember lusamine posing a problem) but i dread ghetsis in any playthrough of BW, idc HOW good i think my team is lmfao. that's a hard ass fight man.
BW Ghetsis for me. I would say Mother Beast Lusamine would be in contention, but I haven’t played Sun/Moon with the exact same ruleset that I used with Black/White (the most notable difference between the rulesets being the fact that Revive Spam was a legal strategy in Moon)