Am i the only one who always fail the In Combat bonus missions?

If you mean the challenges, some of them are a mandatory "screw it" depending on the characters you use, the difficult of the combat, and your goals there. (Have to love when you get the no-healing and no-air damage when playing with Pandora in the party. Or the ranged-kill and all-players-must-get-a-kill with an Astrub knight. Or that kill-all-enemies-in-the-same-round.) The rest is just about focusing into it if you really want the added bonuses. But sometimes the amounts of burnt neurons and defeats to complete them are not worth it.

Depends on the challenge and what you're fighting for. XP grinding? Try your best for 50+ prospecting/wisdom, but don't stress it. Fighting just to harvest their bodies? Just kill ASAP and keep moving.

They really are heartbreaking when you fail them no matter the reason though.

Don't worry too much about it. It happens. I often end up failing it on the last turn...

One tip is to always check the challenge once your turn starts. 

And as someone mentioned above, sometimes just say screw it and go for speed rather than the bonus. 

If you're old enough to drink in your country, take a shot each time you fail a challenge

I'm joking, that's a horrible idea; please don't get alcohol poisoning! ＼(º □ º l|l)/