"As Baron Samedi, cause 3 enemy gods to be above 30 Hysteria at the same time."
Hysteria is Baron Samedi's passive ability. 1 stack is applied every time you damage someone with a basic attack. When you use his abilities on, multiple stacks are applied instead. Vivid Gaze adds 10 stacks per hit (so 20 stacks total if you hit both beams), Consign Spirits adds 20 stacks per hit, Wrap It Up applies 5 stacks per hit (so can get a max of 40 stacks), and his ultimate provide 5 stacks per hit (so a max of 50 stacks).
You have to get three gods with 30 stacks all at one time. At the time of writing, the stacks take quite a bit time to fall off, so if you were to get someone up to much higher Hysteria, as long as you get two others above 30, and the other god hasn't lost all the stacks, then you'll get this award.
The best way to do this all in one is either in Arena or in Assault. Wait until enemies group up and then you need to dump your combo on them all at once. The easiest way is to hit someone with wrap it up to apply 5, then hit anyone near by with Vivid Gaze to apply 10 stacks (if you hit them with both beams, I believe it applies 20, as it counts as 2 hits, but this needs to be confirmed). then use consign spirits to hit them all and them top it off with your ultimate, which ticks every .33 seconds, so applies 8 stacks every tick.
If you use your ultimate and then pull backwards, you enemies will be taking tick damage for much longer, adding even more stacks.
My basic build for Baron consists of Shoes of the Magi, Chronos Pendant, Spear of the Magus, Spear of Desolation, Soul Reaver and then either Telkhines Ring or Demonic Grip.
This achievement will likely come naturally, especially in team fights.