It's very hard to estimate how tough a battle will be in Anima. As has been said, there is a wide range of rolls, and with open rolls, just about anyone can kill anyone with a lucky roll. I've nearly killed PCs (no actual deaths yet) to standard guards, and I've lost major boss villains in a couple shots.
One thing to keep in mind: not everybody fights to the death, and not everyone is out to kill their opponents. If it looks like the PCs are in trouble, you can have your villain take the opportunity to escape, or you can have them surrender, offer to simply end the fight in a draw, or demand the same from the PCs. In real life, people usually aren't trying to kill eachother in fights, and when it looks like an enemy is willing to concede, most people are willing to accept that. After all, even if you're winning; the longer the fight, the more chances for you to get hurt.
This doesn't always apply to monsters, especially if they hungry, or particularly evil, but most rational animals don't like dying, or even getting hurt, so they'll give up when they realize their prey isn't worth the risk or the effort.
Do it right, and it won't even look like GM fiat