Sadly to say, Vine is single target. The only possible mass immobilizing spell I could think of is Encase with chances to freeze due to the Freezing skill (quite effective actually since casting's very fast).
Each Combat Mage build has rises and falls throughout the game which is fair enough. I have a Lightning Mage which made act 2 a breeze (especially in the Vai'Kesh forest) and I know full well that in the end I will struggle against the Qatall with their lightning immunity. Against a not-immune-to-lightning enemy though, the damage output is just crazy with Drown + Multi Spark (it helps my Ranger in using Charged Shot too).
I did mention that Cursing could gets you killed but it's all down to how well at micro-management you are. My method is to get the Mage in, cast the curses and make a run back to my group (in Waiting mode) and turn them into Rampage mode. The NM's Encase/Vine (against big/small group) would hopefully hold the mob in places for free shots. Usually around 3-5 casts of Multispark are possible until the CM has to move again (unless I run into Kurgan, which is annoying as hell with their durability and speed). My other 3 party members are Dual Wielder + Ranger + Mature Lap Dragon so it is always quick work (powers without hesitation
). It doesn't have to the CM dealing damage all the time, I switch between Decay Armor/Drown/Cripple depending on who I'm up against. Drown also turns NM into a good damage dealer (Frost Beam + a few points in Quickened Casting work best for me) since it increases both Lightning and Ice invulnerability (max %55 more, it's no laugh).