I didn't do the main quest until after finishing the fighters guild and the the knights of the nine I was level 18 by the time I took on the first gate and I did fine. The Spiders Deadra were the worst as they had a polarizing poison.
Where I regretted it was the last two big fights Bruma and the capital, because you had a time limit and all those extra enemies where not helping the time limit. With Bruma I just made a speed spell and a very hard lock spell and booked it to the signal stone.
The imperial city though I had to turn down the difficulty, not because I kept dieing but because Martin kept dieing. I really hate escort quests where the NPC is capable of dieing because they always run headlong into the fight like they have hp to spear.
