PedSetActionTree(PEDNAME, "/Global/FIGHTINGSTYLENAME", "Act/Anim/FIGHTINGSTYLENAME.act")
There you go, it will work for all styles, but dont forget to load the animation's needed for that fighting style, aswell dont forget that when using boss fighting style you will need to load all of the animation's needed for them (sometimes they need animations which are more then their ped animations) and set their AI to the boss AI like this :
PedSetAITree(PEDNAME, "/Global/AINAME", "Act/AI/AIFILENAME.act")
good luck