dude Juha can see all futures from all realities, he can then write the present the way he wants, if he sees that Zeref beats him in 1 timeline, he can rewrite reality using a different timeline where Zeref fails to active his power, or kill Zeref before he activates his power
The almighty is the power to rewrite reality, Juha's power is not just precognition the way you seem to mistake it, Juha was pretty much unbeatable because he could see the future of everyone from every reality, he could then choose which future he want to happen in the present