Nipplepwner's solution should help to get you through the majority of the game, and he's deffinitely right about hard difficulty being ok so that only one playthrough is needed.
I just like to add a few things about the final boss fight, since it gave me a lot of trouble at first.
Go into the final battle with full supplies of flamepots, throwing axes and healing potions. The actual battle phase is not very long and consists of three stages. In the first you have to kill three shamans, then three giants (for which you are allowed to use dragon runes) and then take over the portal.Don't use any flamepots, axes or potions during the battle.
After the battle you are on your own and have to fight your way towards the final boss through a number of enemies. Try to get through this as well without using flamepots, axes or potions. Make instead use of your frost rune, espacially when outnumbered and try to throw enemies with (LT + A) over the outer wall or into the lava streams that run down the inner wall. There are three segments in this fight, each with a respawn point in the beginning and with a red dot enemy at the end. After the last segment you reach the final boss.
The first phase plays out a bit like a shaman fight, except that the four rods or seperated by flame walls and take seven hits each to destroy. After you have destroyed a rod you have to wait a while for the flamewalls to drop so that you can reach the next rod. Same goes for the time after the destruction of the last rod, don't run towards the boss, the game will switch phases after a while. The key to survive the big onslaught of spawining enemiesl during this first phase is to abuse the (LT + A) move. By constantly blocking you avoid most of the damage and with (LT + A) you can push the enemies into the flamewalls, which will kill them. Pushing enemies away should always have pirority over damaging the rods, even if you might miss one of the flamewall drops because you take to long for the current rod. Don't worry the walls will drop again. If you need to, drink a porion during this phase. I used one on the third and one on the fourth rod.
The second phase is a giant like battle, and is very easy if you conserved your flamepots and axes. Bombard the boss with the flamepots first and then with the axes. For me it took the three pots and then only 4 to 5 axes, afterwards you start a button sequence again which is different from the usual one. If you should run out of flamepots and axes you have to go in for the rest of the kill, use your usual giant killing techniques then.
Hope this helps a bit.