Can anyone tell me what areas are deemed "high" levels? I'm currently in Lost Knife Hideout and getting my butt kicked by bandit plunders and marauders. I'm lvl. 19 and it seems like the game is getting harder.

Bandits shouldn't be that hard, what sort of build do you have?
If you've focused on something other then a combat ability then you'll have out leveled yourself. By which I mean, the enemy's will get slightly stronger but your skills haven't kept up.

Skill wise I focus on Conjuration, Destruction, One-Hand, and Smithing. I currently am running around in Steel Plate (which seems better than dwarven). I typically use Flame Atronarch and Bound Sword going into a fight and I haven't had that much trouble until this particular cave.

Anyone else have any info on where to avoid in the map till I level up?

That's exactly why dungeons are locking the level. Outleveled? Come back later. It's not like there is nothing else to do.

Wow, funny you mentioned this. I too am Conjuration, Destruction, One hand, Heavy Armor, Smithing. I also JUST cleared that Dungeon last night at, and at level 14 Expert. I've got Lydia with me too. Both Lydia and I are full Dwarven (Superior) armor which is better than steel.

How does your typical fight progress? I usually drop a Flame Atronach to start. If someone is targeting me, I'll Unrelenting Force to stun and then power strike them with my Dwarven War Axe all while shooting fireballs, lightning bolts, etc. There have definitely been some Dungeons that I need to go back to. Swindlers Cave for example. There is a mage in there that one shots me easily. Also, the Felgan cave that the Companions send you to.

Two things,

make sure you make use of the smiting skill and have completely upgraded your armor.

Second, your experience sounds like mine but I managed to make it too about lv 25, grinding through with a lot of potions and I had to stay out of a number of places. Now at level 30 the game is relatively easy again after really concentrating on upgrading my weapon. I put a bunch of points in to enchanting, found a 35 damage one handed weapon and put a 20 point fire damage enchantment on it. That made all the difference. Sorry but I don't know if a conjured weapon is going to cut it.

From 25 to 30 most of that came from leveling enchanting and smiting. My advice is go collecting soul gems and level enchanting. Armor enchantments help alot too. You can put 10% reductions to mana use and such which really helped.

I'm a light armor, one handed weapon, destruction character.