Life gain against fatal spell damage
You would have 1 life remaining. Action order:


Opponent casts Beacon of Destruction, it goes on stack.
Sunscorch Regent triggers, it goes on stack.


If both players pass, the stack starts resolving top to bottom. First you gain 1 life and put a counter on Regent. You now have 6 life. Next, Beacon resolves, you are dealt 5 damage and now have 1 life.