Zhang Yi did not say that they could use the phoenix dignity technique and still have an attack that was twice as strong, with an attack like this Zhang Yi would be able to kill the sixth level demons of the Eternal Realm.
Without the restriction of having to take care of the distorted timeline, Zhang Yi could use this technique four times until his qi ran out, but after that, he could still fight normally with his clones.
So Zhang Yi was confident of winning the war after getting the space Dao, that was because he could help the other beasts to fight and still kill demons from the sixth level of the Eternal Realm.
The biggest problem was that Zhang Yi had to be able to kill dozens of demons from the Eternal Realm for the fight to really stay the same, the way they were Zhang Yi could help a lot in the war, but it wouldn't be enough for them to win.
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