The combat system isn't really explained anywhere, but from observation it seems that the To Hit chance for all monster attacks vs the chars is always 100% minus a 1-2% chance of "crti fail/fumble", and the only way to avoid being hit is to pass a % roll under evasion .
Is it really like this ? seems a bit broken to me....

Yes, the monsters will miss only if you Evade them. Or if their wounds make them fail. Or a few fumble now and then - don't rely on that. It's clearly not balanced compared to your team low hit rate, but you get used to it.

In short, equip armors, build your resistances and wound the bad guys to limit the damage. But you'll take some damage anyway.

If you want to evade, you can build the skill and use the Paladin skill. But there is also an overlooked option : Defend. By using this option, your attack rating is added to your evasion - and that's a lot, enough for any decent fighter to avoid almost anything. You can have your front row evade while your back row hit them. Or strike with DoT (bleed, burn, poison) and then defend. Or occasionally defend with the guy who is close to death the time to heal him.That can make for a long fight but that also can make you win an impossible fight, that's the main use of Evasion.

To add to what Gabby2 said, you can also stun or freeze most monsters, and if you have enough of it, they will get very few chances to even attack you.

I have found it best to stun or freeze whenever you get the opportunity, using bard songs can also help to improve your hit and evade chance a great deal.

Magic attacks seem to always hit and summoned creatures seem to have a better chance to hit than your normal party members. They can also pull ,some of the enemies attack on themselves. So having a summoner and a mage in your party can be very important.