You can just edit them on npc/pre-re(or re)/mobs/dungeons or fields (wherever the boss is)
Example Moonlight on payon dungeon originally:
pay_dun04,120,115,0,0 boss_monster Moonlight Flower 1150,1,3600000,600000,1
Her spawn time is 60 mins - 70 mins
Initial spawn is 3600000 milliseconds = 60 mins
Possible delay spawn is 600000 milliseconds = +10 mins
Which makes it 60 ~ 70 mins
So if you want it 60 seconds - 120 mins:
pay_dun04,120,115,0,0 boss_monster Moonlight Flower 1150,1,60000,7200000,1
Initial spawn: 60000 milliseconds = 60 seconds
Possible delay spawn: 7200000 milliseconds = 120 mins
Which makes the spawn time 60 seconds ~ 120 mins.
Then you can just do this on any other bosses on their respective maps in mobs folder