i have heron and thousand tail watcher on a battlefield, my opponent uses rampage paying its additional cost targetting his empowered ambessa, the wolf, and my heron, i play from hidden temporal breach targetting my thousand tail watcher to reduce the might of his units, can i play a spell later with the reduction from heron?

Yes — but the discount lands on the card you play after the Heron trigger resolves, not on the Watcher Temporal Breach replays.

Temporal Breach is your first card this turn, and it resolves before Rampage, so Heron's trigger fires when Temporal Breach finishes resolving while Heron is still at a battlefield. That trigger goes on the chain, resolves, and your next card this turn then costs [2][A][A] less — even if Rampage resolves afterwards and kills Heron, since the condition is checked when the trigger fires and the pending discount survives removal (FAQ #11829).

The replayed Thousand-Tailed Watcher doesn't eat the discount: it's played during Temporal Breach's own resolution, before the Heron trigger is resolved, so it isn't "the next card played after the ability resolves" (FAQ #12598). Heron's trigger only fires once per turn, so this is your one discount.

One practical check: since you played Temporal Breach from Hidden, its target must be a unit at the battlefield where you hid it (Rule 811.1.d.2) — so the Watcher has to be at that same battlefield.