Im controlling a battlefield with rift herald, the opponent attacks me and kill the rift herald, I want to play a rengar trophy hunter from the deathnell, can i play it on the same battlefield because of "I can be played to a battlefield where there are enemy units"? and if i can play it on the battlefield does resolves before or after the showdown ends?

No — Rift Herald's Deathknell locks the destination to your base, so Rengar can't be played to the battlefield.

Rift Herald reads "[Deathknell →] Play a unit from your hand to your base, ignoring its Energy cost," and that instruction is what's doing the playing (you still pay Rengar's 1 Power). Ambush only adds options to where a unit may normally be played (Rule 822.1.b–c); it can't override a destination an effect has already fixed, and Rengar's unique permission is likewise only a permission, not a replacement instruction. This is the same call as the official ruling on Rift Herald + Ambush units (FAQ #12533).

The timing question is therefore moot — the trigger resolves during the Combat Cleanup, before control is established and combat ends, but it can only ever put Rengar at your base, where it has no effect on the combat at the battlefield.