I get Genesect, and he is so bad, could I do it? I prefer a random one ^^

No you cant. That would make it possible to farm you a good legendary. You can use reroll tickets however to change its nature and ivs

As Djang said, you cannot do that. Once caught you cannot go to Nikola to rebattle, Nikola is only there for if you do not catch the legendary. If you delete a legendary you caught then it is lost. It can be reclaimed by posting in the restore pokemon megathread but that will return the pokemon to you how you had it before deleting it. Also be warned that is for accidental deletions and if it keeps happening it won't be returned.

 

While unfortunately it is impossible to rebattle a legendary once caught you do not have to settle for a bad legendary. There is an IV and nature reroll for legendary pokemon in Vermillion city. Psychic Marina will reroll your legendary pokemon’s nature or IVs. This service is not cheap though and you cannot try for a resync. It costs 150 PvP coins or one reroll ticket to use and the new nature is randomly chosen so please be sure before using it as you could end up with a worse nature.

 

Reroll tickets can be won from the slots at Goldenrod game corner, they are also sometimes rewards in events. The reroll ticket will allow you to change either the nature or IVs of a legendary pokemon once.

 

I hope this was helpful for you and if you have any additional questions than I will be more than happy to answer them.

As Djang said, you cannot do that. Once caught you cannot go to Nikola to rebattle, Nikola is only there for if you do not catch the legendary. If you delete a legendary you caught then it is lost. It can be reclaimed by posting in the restore pokemon megathread but that will return the pokemon to you how you had it before deleting it. Also be warned that is for accidental deletions and if it keeps happening it won't be returned.

 

While unfortunately it is impossible to rebattle a legendary once caught you do not have to settle for a bad legendary. There is an IV and nature reroll for legendary pokemon in Vermillion city. Psychic Marina will reroll your legendary pokemon’s nature or IVs. This service is not cheap though and you cannot try for a resync. It costs 150 PvP coins or one reroll ticket to use and the new nature is randomly chosen so please be sure before using it as you could end up with a worse nature.

 

Reroll tickets can be won from the slots at Goldenrod game corner, they are also sometimes rewards in events. The reroll ticket will allow you to change either the nature or IVs of a legendary pokemon once.

 

I hope this was helpful for you and if you have any additional questions than I will be more than happy to answer them.

 

got it, i'll try the luck soon, thanks