Well I think there are a couple of strategies for beating stage 39 boss for the first time and it's fun to work one out for yourself!
I'd say grinding rebirth points is the most beneficial as you unlock more buff slots, raise your stats and max lvl.
For a high damage, high risk, high reward based run you can choose First strike [T3], Combo and Conquer with T2 formation
For a more tank build maybe you could try Invisible [T3], Blood art [T3] and maybe Conquer as well with T1 formation
I guess T1 formation is good with the blood art buff as it heals you based on your max HP but overall I don't see many situations where T1 or T3 formation is useful.
Anyways, having any more buffs active is a nice addon. Good luck and have fun :D