Death clock doesn't change scenario objectives at all. It just provides a finite, yet known, maximum amount of time you can use to make your moves/attacks over the course of a full game. If scenario has a 5 round limit and neither player times out, then the winner is determined by scenario rules. Which should happen 95% of the time (unless you're running a high-pressure tournament, or have a very short timeframe to complete it in and are setting unrealistical times).
death clock doesn't mean you play to the death (table your opponent), you still play the scenario as always. The reason it's called death clock is you lose if your clock runs out. I've only seen two types of players who allow it to happen…
1.) Players suffering from, "analysis paralysis" - Usually because they don't know the game or their units well enough, or occasionally, because their opponent really put them in a no-win situation.
2.) Players who have zero situational awareness and don't pay attention to their time.
Other than adding an additional lose condition (running out of time) Death Clock has zero effect on scenario objectives or turn limits. The best way to win is to attain your scenario objectives as always, just make sure you don't lolly gag in doing so.
Once typical average game lengths between experienced players are figured out, it's pretty rare to have someone clock out in a tournament who actually has a legitimate shot at winning it. Although, until the right time is found, there will definitely be growing pains. I'll run several, "For Fun" tournaments where all participants get equal awards before running a serious prize one, so I have a good handle on the right time to set the clocks at.
Reactions are going to be mighty interesting if folks forget to switch the clock to the reacting player and back, so that'll be illuminating. Denied made a valid point, but I think the rules handle it well in it not being sporting to give your opponent time to react, it's equally cheesy to hem and haw over whether you will or not when asked. If it becomes an issue, I'll implement clock pausing so it's obvious who's really debating a reaction and who is just messing with their opponent's time.