This event has a lot of green, some red and few purple enemies. Meaning you want mainly red, but if you can’t do red then blue is the second best color.
Given your choices and since you already have Ariel, I wouldn’t ascend Rigard for this event. He’ll be good to do eventually but 2 healers won’t give you much firepower.
Tile damage is king in these events as well, even if you just play to complete the events, not just for high scores. You want to use most of your items at the bosses, which means killing the enemies quickly is the safest way to not take too much damage on your way there.
With that knowledge; it’s between Scarlett or Grimm. I lean towards Scarlett, but since you have Frida with blue-element down, Grimm wouldn’t be a bad choice either.