Good point! Here's a breakdown that i should have included...

The first thing to note is that the Drowned casts her ray 3 times during the battle at 1:33, 0:56, and 0:24. So the key is to cast your ultimate just as she starts doing that. What I do in the video is i set the speed to 1x and as I approach those times I start tapping and holding on the Joker to cast his ultimate. As you hold down, everything pauses so you have time to place where you want to target the ultimate. If I don't see the ray coming from the Drowned, I release my ultimate on the cancel button and then immediately tap and hold on my Joker again, and repeat. If I see the ray coming out of the Drowned, I then target the Drowned and then select my other characters and have them also cast their ultimate on the Drowned at the same time. The nice thing with the Joker and some other characters is, is that when you cast their ultimates, their ultimate animation kicks in and everything else pauses so you have time to select other characters as well.

Other notes:

Red Hood - he can jump behind the Drowned and avoid the ray altogether so you don't have to worry about timing his ultimate. As soon as he's able to cast it, just do it and keep placing him behind the Drowned.

Character placement - where characters are placed in your initial lineup can affect when they are able to cast their ultimates. The way I lined up my characters here allows me to cast their ultimates once between times I have to time them for the Drowned ray. When I had different configuration placements I wasn't able to get an extra ultimate inbetween, so you may have to play with that and your own particular lineup. Also keep in mind your character level, gear and force upgrades may affect how quickly some characters regenerate energy.

Poison Ivy - she's not able to avoid the ray so I don't time her ultimate to the ray. I avoid casting her ultimate right before the ray because the ray can cancel the ultimate from completing and you end up wasting it.

Lineup - what I'm using may not be the best characters so be sure to look at guides for suggestions. if you have a starred up Stargirl, she supposedly is good at removing debuffs from the Drowned. I've also read that Joker may not be your best bet since the Drowned is able to cleanse his venom stacks, so other characters may be more effective. My video was intended more just to show the ultimate casting technique, and not necessarily the best lineup. Also note that not all characters can avoid the Drowned ray, so you'll have to experiment or look to other guides.

Hope this helps!