Answer:the combo rate in depend the fire higher lvl,the lvl number is combo rate.
I quit two year and back in last two month.I know the answer in 3 year ago,i though now people is already understand why cant combo,but seems not,the burst area people keep asking this question getting much and much.
3year ago i was trying to make 17fire to 120lvl reason is i want solo the all instance,and hate wait other people join my inst in server 1.
I have already create the 3 fire same char,same lvl and in the same time,they all have hammer and gs,and 100% 0spd,the burst status is:
1water rb 81+ and other 3 new fire
they burst regular step,and result is not combo,the lvl is all 4 level and the combo is cow 1 by 1 atk and then player 1 by 1 atk the beetle.
After player 30lvl,the player or cow is 2 by 2 atk(cow 2combo or 1 cow 1 player combo).
After 50lvl it have sometimes 3 player or cow in combo.
When they all 100lvl they are all 100% combo.
1st team three fire is already 120 lvl in the next 3 new fire i am already set,got hammer,got gs so start the next new team.
This time i gonna make my main join 81+ rb fire,team status is
1water 81rb+ 2 new fire + 1fire rb
the new fire is 4 level also,and the combo is also not 100%,after they are 40lvl the combo is 100%
So i make a conclude,the max combo rate is 100%
and the highest fire lvl in team is the combo rate(ex:100lvl=100% combo rate)
and the the player atk low will decrease the combo rate,but not so effective.

I guess you're kind of right, but not completely. I found this guide to be the most useful on how to combo attacks.

http://forum.wl.igg.com/viewthread.php?tid=6032...

aside from speed,
level of attackers (including cows) and level of mob(lower the higher chance)

if your fires are high enough, then you should move to a higher mob where you can still combo 100%