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Just beat it with randoms, third attempt after a failed attempt last night and another failed one this morning before I had to leave home.
Expected the Water Tera guy (hidden behind Mewtwo) to be a useless retard because he was spamming Surf at first, but halfway through he got a clue and started using Life Dew and Struggle Bug almost exclusively, making him not complete dead weight.
Me and one of the Japanese Mews were cycling through Misty Terrain and Elec Terrain to prevent it from using Rest at any point past it's shield being down (the same mew also used Light Screen alongside Life Dew and Struggle Bug), me and another jap were the SD Leech Life attackers, beat it with about 35% time left.
Looks like this raid is perfectly doable when all of your teammates are at least baseline competent. Maybe I just lucked out.