There are a total of four divine beasts in The Legend of Zelda: Breath of the Wild. Although they are similar in design, they aren't all equal in difficulty.
We suggest doing the bosses in the following order:
- 1. Windblight Ganon - Rito Village, Top Left of Map
- 2. Fireblight Ganon - Goron City/Death Mountain, Top Right of Map
- 3. Waterblight Ganon - Zora's Domain, Right of Map
- 4. Thunderblight Ganon - Gerudo Town, Bottom Left of Map
Although the first three mentioned bosses are reasonably similar in difficulty, Thunderblight Ganon is widely considered to be by far the most challenging to beat.
Note that Waterblight Ganon is the most accessible in terms of location given Zora's proximity to Kakariko Village. You may choose to do this first if that seems reasonable to you.
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