It's quite easy to beat it, I beaten the game without using any items well as much items maybe 10 at most.
But basically what you do is, you try your best to reduce the boars to just 1, then you regain focus/engery, then once you finish the fight to use skills to heal yourself and repeat.
There will be moments were you go oh I need to heal, just use serume and save burgers for boss battle.
Boss battle is easy as well, All you do is use special attack once, if he charges his attack then use evade, if he doesn't you can use special attack one more time. But after you dodge his special attack you need to regain energy by using focus. At most you will use 2 burgers and 1 serume.