Well most of Bosses has a pattern, like for example Massacre ( juggernaut boss on poi) appears like 1 time per week) so you will have to follow the time of the last boss killed and try to check it more frecuently on the days that you will know that its gonna to appear. There is a fansite of tibia called Tibiabosses that gives you info on many worlds about last bosses killed and the probability of each one to appear based on the previous patterns that I have named of.
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