I played this following the guide Rafael D Arroyo posted, which was most helpful (and will get you all the other achievements too) until my save got corrupted just after arriving on the second map - thanks xbox.
Not wanting to risk another single save file run I googled a bit and found this great guide to speed running honor mode on Steam by "Chrest" https://steamcommunity.com/sharedfiles/filedetails/?id=74652...
all credit for the solution must go to them, I'm just linking it here because it is useful.
Turns out that dropping things like barrels on people will damage them, the heavier the thing is, the more damage you will do, so if you load up one of the unbreakable chests (like the riddle chest you can find near the quartermaster office) with lots of barrels you can out right kill just about anyone without actually entering combat.
Chrest's guide will spell it out in more detail giving the exact location of chests (you need about 70 by the end to one shot Leandra) but the basic idea is to have one character running around with with Walk in Shadows, Winged Feet, Fast Track and Telekinesis. While the other one stays still massively overloaded with a stupidly heavy chest. When you get to an unavoidable combat situation you transfer the box to you mobile character, drop it, go invisible and then telekinesis it on top of the enemy.
It takes only about an hour or so and you only need to go up a handful of levels to max telekinesis, I got it first try using this method as even if you exit invisibility at a bad moment or step on a trap your other guy is stuck at the entrance to the area well out of harms way, a quick teleport back to the end of time will group you remaining guy with the corpse guy to resurrect them.
You will only have to kill Loïc in Silverglen, one demon walking around in the phantom forest, the big guy guarding the source temple door, Trife, Leandra, the death knights near the void dragon (out of combat with the wand) and the Void dragon itself.
This is also an easy way to get the lone wolf achievement.