Okay, watched a video and I think I have a better understanding on the chain battle. Mini guide below:
1. Choose your units. Get ones that hit multiple categories if you can, especially for main attacker, and then units that will link well with your chosen main attacker
2. Get sub units that hit categories as well, but mostly you'll be using your friends units, and one of yours. Goal is to get the chain link filled for both Super and Extreme
3. At the midway point, choose 3 randomly selected units. Get good links but also the faster you choose the more damage you do
So for me, SUPER only had two units hit Majin Saga/Fusion/SSJ3 and that's PHY and DEX Gotenks. After that I kept Majin saga and fusion.
For EXT I had no Majin and fusion or Majin and SSJ3 so I went full Majin saga with some Buus that were red (still don't understand that part)
Went into battle, chose GoBros since I've been leveling their skills during the story event (2/2/2/3/3/2/1 -- so not even close to max yet).
For supports, there's lots of options. Some had fusions, some had Gohans. I went with Oops-All-Gohans and my STR Gohan lead which maxed the chain. I couldnt use the same connector again until reset, which stinks but I get it I guess. EXT came very close but did not max
Attacked, and for selector I saw 3 Buus and just chose them together.
Overall score: 41mil
My high score was 56, which is probably because I used the best connectors already. After reset I'll try again and maybe score a bit higher
EDIT: Ran it again and got double chain max (gohans and buus with 3 gohans as random fast selects). Score was 68 mil. So definitely get that bar maxed out! Currently in the top 2%. Looks like 70mil is 1%
