Finally powered through the dragon gym at 15/16 badges. I tried to use a Cloyster for ice, but I kept getting sick of trying to level it, and it's so slow. And the gym had a Kingdra anyway which is ONLY weak to dragon. Besides, I had Raikou with Thunder and Spark which would work for the flying dragonites - I forgot that dragon actively resists electricity.
Made it anyway, with a combination of Raikou Xatu (Natella) and Houndour (Cerby). Raikou and Xatu were even fairly underlevelled.
Then I finally beat the water gym, which I'd saved for last/full power. It was almost *easier* now, because Misty sorta dropped the pretense of focusing on water. There were like 3 ice, a quagsire, and yaknow just Lugia hanging out as flying/psychic. Fortunately my overlevelled Houndour is very good against ice types and psychic types. Sorta disappointing... I even put Sunny Day on Raikou to set up Celebi with Solar Beam, and I only got to use it on the Quagsire really. Still felt good, depowered the water attacks, and empowered the flamethrower.
So at that point the Elite 4 was pretty underlevelled, which is fair considering it unlocked at 8/16 badges. But there was one sticking point... A Blissey, bane of Pokemon GO and now Crystal Clear, with:
Soft-boiled
Heal Bell
Light Screen
Ice Beam
carrying leftovers. And enemy pokemon have unlimited PP in this gen, so it happily kept spamming Soft-Boiled to heal ~2/3 health whenever it was significantly damaged.
I tried... various things. Flash doesn't reduce accuracy for self-targetting moves. Psychic reduced its SPC Def once, but never again, and the light screen was practically constant and extreme. Spamming Flamethrower and Crunch were complete no-goes, doing about 1/8 health a turn. Raikou managed to paralyze it twice, but it just Heal Belled (never lost a turn to it either).
I was about to try ice beam from my pathetically underlevelled Cloyster, which could barely survive Blissey's ice beam, when I remembered my Marowak with thick club. Even lower level, but it freakin worked. Might have died from a single ice beam, but the Blissey kept trying to heal or light screen instead. Hehe...
Pokemon Crystal Clear - cleared!