when i look for assistance, do i have to ask permission to the assisting player? i mean, he can get some damage if i fail.
The rules suggest that you don't have to ask permission but this is a co-operative game after all so I would consider it bad form if you ignored the other player's opinion.
Friendly minor rant Please do not use the term "locked" when placing die on Spell cards or when Assisting or Focussing, please use the term held for these situations. Locked has a specific meaning in the game.