I've had luck (after countless tweaking) with Squad 1, Squad 3 with a Speed of 313 and 316 (i think it must be 311~312 at least, but i'm not sure: 313 is safe; Speed shouldn't be more than 320), Squad 2 and Squad 4 with a Speed of 556 and 560 (should be 550~560, but should be no more than 560), and the slowest helper you can; this way, you're guaranteed that Squads 1, 3, H will turn right at the first fork (they barely make it, but they make it), and being slower they give you more time before one of them finishes the map, if they go towards the end right away; Squad 2 and 4 will circle down south, and hopefully make it in time to the center, being faster.

The 311~313 min Speed is sort-of the minimum to reach the first fork in two moves; the 560 maximum Speed is sort-of the maximum to not reach the first fork in only one move (probably could go a little bit faster, but i didn't want to test my luck further).

This said, it still took me a total of ~10 runs, but i got it second time with this setup and some luck. No guarantees it will work if the RNG really hates you though (in fact, i still failed the first), but it's as close to minimizing the random-factor as i could make it.

EDIT: As a sidenote, i have no clue about the Squad Power required. The Boss in the middle seemed to deal damage based on % of current HP, but other than that, all my Squads had 30k+ Power and breezed through the map. I'd probably say 15k~20k Power or so each Squad, just to be safe.