ok so i hit 20 hard to find a nice spot for lvl 19, i sugest to stay in the barrens for horde(oh btw in undead mage).
the lions give nice xp till lvl 19, u can also try the plainstriders that are agressive, between boulderlode mine and the sludge fen thats where i did the last 50% of lvl 19, slow spawn yeh i know.
Fenris isle/keep u dont wanna try at lvl 19 u get owned or maybe i just suck :p
at lvl 20 i still think its pretty hard at fenris but just need to fnd a nice route to pull yet.
think ill respecc and have cold snap at lvl 24 instead of lvl 20, and get imp blizz first... we all make mistakes /cry (??)
died over 100 times today just trying to find a good spot.
beren's peril is nice but hard to pull a nice group of undeads there and not a lot of room to succesfully kite...so after 10 deaths i gave up there.
tried the riverpaws at westfall till i found out they backstabbed me to often so i died couple of times there also.
Ravenhill is still bit to high at lvl 19 , i would suggest lvl 22 here at least.
yeh what the hell was i doing there as horde, just trying to find a nice spot!!!
took me a lot of time to realise barrens is good to grind to lvl 20 maybe even to 26 think ill check southern barrens if i die 1 more time in fenris isle...
so aoe mage from lvl 16 to now i like it alot, maybe someone with bit more experience can comment on cycles and stuff, cose i had to (semi) twink myself to get enough mana and health to survive at this lvl...sta enchants on everything possible, yeh even clefthide legarmor kit on noob legs :S, and minor speed on boots, it greatly helps but my advice if u just getting started and dont have a high lvl main to support other chars, dont try aoe grinding at lvl 16 already lvl to at least 20-23 so u have max imp blizz.
at lvl 19 i have 1k9+ hp and 1.4k+ manaand stil i ran oom at the end, just barely killing the mobs i pulled, arcane explosion isnt the most mana efficient way ;) getting smoked sagefishe helps a bit.
hope this info is usefull to someone, and will update later on other spots i found to grind