Obligatorily I have to link
http://www.mmo-champion.com/threads/...who-needs-a-DK
If you're talking about what mythic-geared mages are doing, they're almost killing Garrosh.
But if you're talking about what a ~610 ilvl mage can do......Firelands shouldn't be a problem, but you
have to read the dungeon journal first. DS should be fine too, but I'll be careful on Madness.
If you take the time to upgrade it with Balefuls to ~675, then Elegon 10 Normal is possible, at even a lower ilvl if you can get a 4/6 lockout. I would not recommend Horridon unless you have some HFC gear, or really knows what you are doing. Ji-Kun itself is really easy, but the bosses before it are not, so around HFC gear. Assuming your purpose in soloing is for mounts, of course.