Destroying Odric, Master Tactician in response to declared attackers
Abilities written as "Whenever X, Y" are triggered abilities, they go onto the stack when the trigger condition, in this case "Whenever Odric, Master Tactician and at least three other creatures attack" is met, the ability's effect, in this case "you choose which creatures block this combat and how those creatures block" is put on the stack.

Once an ability is on the stack, changes to the board itself don't affect that ability, it is going to resolve unless countered. So unless you Stifle the ability, removing the Odric, no matter where he goes, be that to hand, exile, graveyard or even flickered back to the board, will not stop his ability from resolving. The only change to boardstate that can affect a spell or ability on the stack is if that ability is no longer legal, say removing all targets, causing the spell or ability to fizzle (be put in the graveyard as it would resolve).