Flashbangs aren't currently useable, their functionality will be added on R36. On R36, you will also be able to sleep once you finish 3 side quests at the Guild.
However, Minotaur fight should still be doable: You can purchase elixir from the Blue Pharmacy (Unlocked after their side quest) which refulls your entire health, and you can also train your skills at the skill shop if you haven't already. Skills are important to face stronger enemies than you, so the sooner you maximize them, the better. I managed to defeat the minotaur and survive with just 10 hp on my first try by having just 1 point on the first skill to raise Riposte's damage.