At stage 1-3 each spins just cost 1 stamina so 250 stamina is even more overpowered there. Chance of 3 time "Chance" seems same at all stages at 1 in 12 spins. Staying at stage 1-3 makes it rellavty easy to get a ex-wild maybe 50% chance per day with out spending items.
Top 5 in the event get 5 *Ex-wild but I dont know it Ex-wild really matter or not. Maybe the advantage of stage 1-3 is so small compaired to what we get from events that it dont really matters. I hear that player in Japan version end up with tons of Pero and nothing to spend them on.

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Just some extra info i discovered

The boss we summons have a level between our Boss-Slay-Count and half our Boss-Slay-Count.
So if you killed or helped kill 100 bosses you NeXT boss will have a level between 50 and 100.
Bosses under level 100 never have 8,7 M power.

If you boss kill Count is 200+ all bosses you summons will have a level between 100 to 200. Because almost all level 100 bosses have 8.7 M hp or more players will get them 90-95% of then times. Rest of the time they get bosses with around 2 M power. This just explains what most players have seen in this event.