Including this card in your deck is like ripping off a band-aid.
At first you look at the 5xp cost, and the net effect, the requirements to pull it off, and careful consideration should bring you to the conclusion that Monster Slayer is a bad card to invest in for 5xp.
I tried Monster Slayer, gave it a chance rather then going straight for Lightning Gun, a card that has proven to be worth the cost. At the time I had already finished a Talent based build built around Well Prepared. And then I got the second copy one scenario later.
In the final scenario me and my teammate proceeded to draw two big monsters in the same mythos phase, and I, like Superman grabbing a couple of crooks and bonking their heads together, defeated them with just one action each.
So there I sat, the monster hunter with nothing to do anymore, in disbelief over how effective the "bad card" turned out to be.
So, yes. Including this card in your deck is like ripping off a band-aid. It looks iffy and you really don't wanna do it, then you finally do it in a burst of stress and determination, and then you relax with a sigh of relief because it was'nt nearly as bad as you thought it would be.
I'll just say this, when you have cards like Physical Training(2xp) and Well Prepared to boost your fight stats without weaponry, Monster Slayer becomes a far worthier addition and a relatively good fit into the cost-curve of such a deck.