Since they've remade the official forum, so I can't find about the old guide about combo attack.
So can we make a dicussion to get clear information, then I can write a new guide ?

This is my information.
To combo-attack , it requires :

► Find the average level of team : M (who's going to attack the target)
► Level of the target : N
► M - N must be greater than 25
► Follow the "chaining speed" or "speed chain"

I did these thing, but I didn't do the combo-attack. I really don't know why in my case.

char a - level 68rb - speed : 346
char b - level 109rb - speed : 321
pet a - level 70 - speed : 270
pet b - level 80rb - speed : 380

target - level 100 - speed (as my calculation) : 288

M = 157.5
N = 100
M - N > 25 (done)

speed chain works (done)

The average speed is 329. It is also greater than the target's speed (288) .

But my team can't combo. Please explain me.
P/s : Hmm.. I think all speed in team must be greater the target's speed. But pet a is not (270<288).
I wonder if it was the reason.