Where is this trailer ? Also actually no, Focus Blast usually doesnt OHKO Hydreigon as its rather bulky, depending on the evs of course. :x
I have a Non-SP. Def EV trained Hydreigon that doesn't have a deliberating ability and he ALWAYS gets OHKO'd by Focus Blast when it hits unless the Pokemon using it has a base power less than 90...I know my friends Gardevoir can one shot my Hydreigon with it. I know I got one shot by an Electivire as well it as a base of 95 and favors physical attacks over special attacks. A Toxicroak came close before as well.
According to Psypoke's calculator a Croagunk could indeed nearly KO with base info only depending on if my nature and EV's effected my HP, which they didn't. The curve was 248 as the lowest and 294 as the highest. Hydreigon has between 294 and 388. I don't believe mine has many as it was SP. ATK and Speed trained with remainder going into HP (can't remember if HP or Def) and its IV spread is pretty average. I survived a possible one shot so assume it was.
(have to use Gengar and Mime. Jr but lowered Gengar to 125 SP ATK using Ominous Wind since its assumed Fairy Wind will match)
Damage output was between 182 Minimum and 216 Maximum. Hydreigon with no EV in HP would be 294 which would be around 62% to 73%.
With two weaknesses...(changing to a bug to use Silver wind on a Grass/Psychic)
Minimum damage was 244 to 288. That would be... 83% - 97%. Which is more than what was shown. Average hit would be in the 90% range...
So it really doesn't look like it was x4 SE damage. Psypoke's calculator (allowed me to put proper attack and defense ranges thank you holy figure) disagrees with x4 damage doing so little damage. to a Pokemon such as Hydreigon.
No IVs or EVs added at all.
I made an oopsie. I forgot to change Gengar to a bug type so the previous calcs didn't add STAB.
x4 SE hit with a base 60 does 364 - 432. Well over Hydreigon's HP levels and around 90% on a fully supported HP Hydreigon)
Went into Smogon's calculator and got similar outcome. Numbers are different but the damage is near the same point.
Bug with 125 Sp. Atk. Using Silver Wind. Grass with HP 92 and SP. Def 90.
x2 SE Damage came to 180 - 212 55.4% - 65.2%
x4 SE Damage came to 360 - 424 110.8% - 130.5%