I just beat Honour Mode and thought I could write a fews things that helped me getting the achievement.
First of all, GREAT GAME, I can't say that enough, easily the best game I've played this year.
Okay, after losing two saves with at least 5 hours in each of them I went looking for help and found this exceptional Gameplay Guide by gamer Kallume:http://psnprofiles.com/guide/3793-Divinity-Original-Sin-Enha...
It's for the PS4 but it works perfectly for the One version and it's a must use, it outlines with great detail the steps you have to take to earn all achievement in one playthough and the most efficient way to get experience and level up, making most sections a breeze. Try to read ahead of it, to prevent dying because of a wrong dialogue option, for example.
(Both the fire resistance lava and cloud save tricks dont work on the XboxOne, but they aren't really that necessary and the cloud save one is basically cheating).
The second important thing is PATIENCE, always explore with only one character, and bring the rest of your party only when you know the path is safe and you're going into a fight, your worst enemy at the beggining of the game are TRAPS, since in the late game, when you're stronger, traps won't kill you instantly and you only have to worry about lava. If your "explorer" character dies, just revive him, no need to worry.Following the guide I linked to the letter, the only close calls I had during fights where:
All the fights where you have bombers (usually at the first hours of the game), they'll explode and probably one hit kill you, when these fights start immeaditily deal with those pests.
The fight against the giant spider at the end of black cove (this is where I lost my first save) - if you can't immediately freeze/stun/knockdown/etc the spider just reload the checkpoint, trust me, you don't wanna risk it.
The fight against Jarreth at Luculla Forest Sacred Stone - He starts with many allies, and both him and the bull he summons deal great amounts of damage, so what I did was keep reloading untill I successfully cast charm on the bull, then it goes on to destroy the other enemies, and you only have to crowd control Jarreth.
Gruntilda at Hunter's Edge, not a hard fight, but if she starts attacking one of your characters, reload, she deals a lot of damage and was able to kill one of my character with 3 attacks.
On the Phantom Forest beware of the EYEBALL SENTRIES they will kill you on contact, dont even get into a fight with them, kill them before the fight starts, because it has more initiative and you will die
, I had one kill 3 of my characters and left one main character alive with 10% health, that was the closest call I had on my successful playthrough.
*Ideally you will never have a character dying during a fight, because when they die the game immediately saves and if both your main characters dies, goodbye save*
Strangelly enough, the last three boss fights of the game where a breeze for me, the Void Dragon is annoying because you have to protect one of the NPCs, but the fight itself is very easy, he never got anyone's health bellow 50%.
I guess that's it, I'll update it if I remember anything else, but Kallume's guide should be enough, it's a really well done guide and helped me a lot.
For those of you that dont want to feel the joy of utterly beating this game and just want a 100% completion, I still have my checkpoint, and am willing to share it, but only if you have the game completed on tactician (It's the same difficulty level and I guess anyone who has the game beaten on tactician deserves the achievement).