First of all: SAVE YOUR GAME AFTER FIGHTING THE DEMON. After fighting the demon and then talking to the grand enemy, make sure NOT to ally with him otherwise you will not fight him
What I did for this achievement was put the game on Easy and just explored the map(didnt get all of it, but uncovered a lot of it) so that I could be properly levelled(I wasnt quite sure how hard the game was) and so that I could find the location 'Ashos' and then I followed this link: http://www.xbox360achievements.org/forum/showthread.php?t=36...
Once I was at that point I just got all of the other miscellaneous achievements minus some location achievements and level 50 and ended up at level 37 when I was at the boss.
if you must fight a mob of powerful enemies like "Giant Trachidis" with melee(I only used melee) then I would try to lure them near a respawn point, and then if they killed me I would just spawn right next to them. I do recommend fighting "Giant Trachidis" though, because each one drops and egg that raises vitality permanently.
I also recommend maxing out air magic(if you are a melee user) and then finding the "Strength of God" spell. I cant tell you where it is because you can randomly find it on a vendor or in a chest(I found it in a chest). The spell will increase your damage by an INSANE amount, which along with the weapon from ashos, will just tear through most enemies.
Do not be afraid to use your mana or health potions in a tight spot. If you are far away from a spawn point, then go ahead and just spam your potions. I ended up using my health potions all up and I was fine later. I just found more and kept going.
For the Demon(before the grand enemy): lead him to the spawn point and just fight it out. If he actually kills you you will just spawn right next to him and continue your fight.
Overall the gameplay was much easier that I thought it would be. Just level up as best you can, and you dont have to use the weapon/spell I recommended, It is just what I used. You can use the weapons or spells that work best for your style of gameplay. I would say being level 30+ should easily get you through the story(level 30 is not too hard to get). If you really just want this achievement and dont care about anything else, you could get by with 20+ just some enemies will be harder, but enemies dont get their health back, so keep that in mind if you are worried about being killed.
Good luck, and I hope someone was helped by this solution.