hello I am a new runner of this game and I have many questions.

I am constantly getting better, but my times are not diminishing. I keep dying until the last boss. he takes too many shotgun rounds to kill and eats me.

maybe there is a solution to my problem?

Greetings. : )Which category are you running? Usually the bosses in leon scenario are done with the magnum. There is a strat for final birkin that the most runners do. I suggest watching some runs from the leaderboard on that boss fight. Its not that hard to do.