So in a game this evening I had a win stolen from me. I'll explain what happened and ask how the general community feels about this. So my opponent had two Imperial Walkers out, one damage on an objective with a capacity for five, another objective with three damage with a capacity for five, and another with no damage. I had already captured one objective. On my side I had four X-wings, and a Y-wing. One of the X-wings had one damage on it. I took that one and my Y-wing against the objective with one damage on it. My opponent declared one of his Imperial Walkers as a defender and didn't place a shield token. I won the edge battle, and resolved the X-wing first, doing one damage to the Imperial Walker, and one damage to the objective. Immediately after I played Heroic Sacrifice and used the damaged X-wing to eliminate the Imperial Walker. Now after his unit was removed, and I was about to resolve the Y-wing, and he declares that he is shielding the objective that is about to be destroyed. His argument was that it is an action that HAS to happen, and refused to listen that there is no text stating it is forced. He also stated that he was going to put it on there anyways. My thoughts are, he saw it as the card was being removed, knew it was pointless to put it on the unit, and placed it on the objective to avoid having it captured.

I think that my biggest question(s) to this is, is this seriously allowed? Is the shielding effect a FORCED deal even though it isn't stated any where?

Are you asking about the "Shielding" keyword? From the rulebook, P.25, "When a card with the “Shielding” keyword is declared as either attacker or defender, its controller may immediately place one shield token on any friendly unshielded participating unit, or to the engaged objective card, if unshielded."

As it states "may immediately place", it is an optional effect, and if used, it must be immediately after the unit is declared as an attacker or defender. If it passes the chance then it is lost. Your opponent is wrong in this case.

Even if it were a forced reaction both you and your opponent have taken meaningful actions and shouldn't be able to go back

right. Even if it was passive, and required (which it is not... it is optional ), him not realizing it till you've already committed to destroying his only unit in the engagement does not give him the right to take-backs and be like "oh, I meant to put this on the objective." I have the feeling he had no idea he needed to do that, since his Walker could easily have blown up your Y-Wing before it struck, saving the objective anyway.

Bad form on his part. He's just a sore loser, I suspect.