If I have a Watchful Sentry at a battlefield and my opponent attacks and kills my Watchful Sentry, if I play Rengar Trophy Hunter in response to the deathknell trigger and win the combat, will I conquer the battlefield again and get a point?

To answer your question: No, you will not conquer the battlefield or gain a point in this scenario.

Here is the breakdown of why this occurs based on the game rules:

1. The "Once Per Turn" Restriction

According to Rule 465, a player may only score (via Hold or Conquer) once per battlefield per turn. If you have already scored at that battlefield earlier in the turn (or if the game state otherwise prevents a new score), you cannot score there again.

2. Conquer Requires Scoring

As established in FAQ #1689 and Rule 466, "Conquer" is a subtype of scoring. If you cannot score (because you have already scored that battlefield this turn), you cannot conquer. Even if you win the combat by playing Rengar, Trophy Hunter and clearing the opponent's units, the game will not trigger a "Conquer" event because the scoring limit for that battlefield has already been reached.

3. Combat Resolution and Conquer

Even if you hadn't scored that battlefield yet this turn, the sequence of events is as follows:

1. Deathknell Trigger: When your Watchful Sentry dies, its Deathknell ability is added to the chain.
2. Reaction: You play Rengar, Trophy Hunter as a reaction to that trigger.
3. Combat Continues: The combat showdown remains open until both players pass priority. If you win the combat after the chain resolves, the game proceeds to the Combat Cleanup (Rule 461).
4. Establishing Control: Per Rule 461.5, if you are the only player with units remaining at the battlefield after combat, you establish control. Establishing control results in a "Conquer" only if you have not yet scored that battlefield this turn.

Summary

If you have already scored that battlefield this turn, winning the combat with Rengar will allow you to establish control of the battlefield, but it will not trigger a "Conquer" event, and you will not gain a point. If you have not scored that battlefield yet this turn, you would successfully conquer it and gain the point.