Dragon Priests are easy for me.
My Light Armor is around 85+, and my One-Handed is 80+.
I'm using dual Glass Swords (Epic) and all I have to do is use Elemental Fury.

Hope that helped, guys.
The main thing is getting close to them, or avoiding their attacks. Getting Frost\Fire resistant armor is a good idea. Also, Marked For Death softens them up a lot, making them extremely vulnerable to attacks. And finally, if all else fails, Ice Forms will freeze their in place for a while, so you could run up to them and just kill them. Elemental Fury doesn't work on enchanted weapons.
around 40 now. It doesn't matter if you can't kill them before they do the same to you with their destructive AOE spells though.. Both forsworn briarhearts and dragon priests have been such a pain in the ass for me, they are harder than dragons themselves, hands down. They are even the hardests of things I've confronted by now, I can tell. Even if you have a high armor rating, they will deal extreme damage to you like in no time and you can find yourself dead before you even realize that you will die in the next second. However, I've found a way to counter this as well, if you are a good restoration user (you must be, if you consider yourself a pro though) ward spells come really handy in there. I didn't think that ward spells might have been that useful until I tried it out with a briarheart. It makes you almost invincible to spell damage provided that you have magicka (well, the magnitudes of ward and spell damage matters though since ward spell can only reduce a limited count of damage, so I advice using the best ward spell possible)
Those two enemies, plus mages and Falmer Shadowmasters are the only enemies I've ever had trouble with in the game, yet all of them fall fairly easily to Marked for Death and a few dual-wielding dagger attacks, or bow shots. Sneaking is just too overpowered and doesn't let you increase your armor skills, so the advice I'm giving is only when you cannot sneak. Because sneaking WILL kill EVERYTHING in one-two hits. Either way, if mages give you trouble, get frost\fire\storm resistant armor, preferably with the Two Enchantments perk.
I'm 30 now, i think i was 28-29 back then. I'm playing on adept, and he still killed me in 2-3 lightning hits.
Well, that's a really low level to encounter Dragon Priests.

I found my first ones at about level 40-45. Make sure you have some good armor(preferably Dragonscale\Dragonplate\Daedric with enchantments, depending on your primary armor skill). Also, constantly move and evade their attacks, or hide behind pillars. I find lightning attacks really hard to evade, though, since they seem so... Accurate, and instant.
Well, levels don't really matter, because they scale with you. The
difficulty does matter. It's been a long time since I fought one of those priests, but I remember him killing me in like 2-3 hits (on master). I always summon something when fighting those bastards.
I remember reading in the wiki somewhere that level
does matter, and enemies
do not level with you. Instead, you just encounter more powerful varieties of monsters as you level up. (Falmer --> Falmer Gloomlurkers --> Falmer Shadowmasters, for example. Or, Draugr --> Draugr Wights --> Draugr Scourges --> Draugr Deathlords)