Replace Derby code in the ide.img with this one:
PRlead_Darby....PRlead_Darby_W......Large...PREPPY..STAT_P_BOXING...F_DARBY.Evade_Duck..P_Darby.Boxing....../Global/BOSS_Darby..Act/Anim/BOSS_Darby.act./Global/DarbyAI.Act/AI/AI_DARBY_2_B.act.N_Darby
Or create a custom lua file and add this line:
PedSetAITree(37, "/Global/DarbyAI", "Act/AI/AI_DARBY_2_B.act")
Instead of the number 37 you can use a local.