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Yo, wut it do my fellow autists? I was relishing in my prediction about the Deino Community Day, when the idea came to me "How good will Hydreigon actually be in raids?" While Hydreigon will probably still be at best glass cannon shit in GBL (assuming that Brutal Swing is only 35 energy, aka genie-tier wish), there is a significant chance that it will be much better than it currently is in PvE.
After flipping though some frames of various videos of Brutal Swing's animation in Sun and Moon, I've determined that it ranges somewhere between 1.6 seconds and 2 seconds long. Assuming Niantic keeps their animation fairly consistent. I have input various possibilities for the stats of this move (aside from damage, which we already know), variables including duration of the move and amount of energy needed to use the move (Key: "[Duration] + [Energy Cost]"). Pic related is the result.
As you can see, all calculated possibilities will be better than shitty Dark Pulse. On the top end (1.6 sec + 3-bar charge move), Hydreigon is arguably strictly better than Mega Absol, so we probably won't get that. Second place is more realistic, with a hair less DPS than Shadow Ball Darkrai, but with significantly more TDO. Third place is when we enter cringe territory (65 power for 2-bar), but Hydreigon is still just slightly better than Mega Houndoom with this moveset. Fourth place is worst case scenario, and even then it's a small-DPS-for-big-TDO trade with normal Weavile, so definitely still decent for newer players.
TL;DR Hydreigon will be significantly stronger, but probably not the best.