I have a level 22 mage, with talent points spent soley in the Arcane line of magic. With mobs equal to or one level more than mine I can usually go through three or four of them before I need to drink anything. I find when I pull single mobs that it is usually better to use fireball than frostbolt to pull them because of the added range (5 yards doesn't seem like much but it's a huge difference). True, frostbolt does slow them, but there's a chance it will be resisted, and depending on the mob you are pulling, it might activate a speed spell that negates the slow effect anyway. Also, the fireball deals more damage, an important factor since the faster you kill the mob the less damage they do the mage (they don't exactly have the best armor). Once I have pulled them with the fireball I use another fireball to maximize damage before they reach me. From here I slam them with two full sets of arcane missiles. If you need, you can use frostnova first before they get to you to avoid damage, and use the four to eight seconds to hit them with the arcane missiles. Now, the next part depends on their remaining health. Some mobs will attempt to run away, which gives you an awesome opportunity to use your wand. Doing this will recharge some of your mana since you won't be spell casting. If the mob doesn't run away, or if their health isn't quite low enough for them to run away (i.e. if they resist some of your spells), you can fireblast them to deal some quick damage, and then use your wand to finish them off.
If you are flying solo I don't recommend taking on multiple mobs back to back, but if you do find yourself fighting two or three mobs without a full mana bar there are ways to come out on top. First, assess your remaining mana. If you've got 20% of your mana and the mobs are around your level, running is the best idea. If you think you can do it though, then take a quick inventory of your enemies. You should polymorph one of the enemies first, preferably a ranged unit if there is one. Ranged units can still attack while frozen, so get rid of it so you can fight the melee mobs. If you don't have a full mana bar then you probably don't have a full health bar either, so protect yourself first. If you die then your remaining mana doesn't matter. Frostnova the melee units, using fireballs for single units and arcane explosions for multiple opponents. Try to finish off the remaining units before the polymorphed enemy changes back. If you have a few precious seconds before you need to fight again, use it to either get away from the sheep so you have some distance, or if you desperately need the mana you can use evocation.
Now, it's a good idea to keep some potions with you at all times for emergencies. You will sometimes need that extra boost of mana or health. If you can, add some armor, intelligence, and spirit with a good combo of potions, scrolls, and other buffs. This will help you keep going without having to stop.
I hope this helped! I know there are other ways to go about your magery, and this might not be the best for you, but it's the best way for me so I thought I'd share in case it could help you.
Happy Hunting!
-FluffyBSFG
If you are flying solo I don't recommend taking on multiple mobs back to back, but if you do find yourself fighting two or three mobs without a full mana bar there are ways to come out on top. First, assess your remaining mana. If you've got 20% of your mana and the mobs are around your level, running is the best idea. If you think you can do it though, then take a quick inventory of your enemies. You should polymorph one of the enemies first, preferably a ranged unit if there is one. Ranged units can still attack while frozen, so get rid of it so you can fight the melee mobs. If you don't have a full mana bar then you probably don't have a full health bar either, so protect yourself first. If you die then your remaining mana doesn't matter. Frostnova the melee units, using fireballs for single units and arcane explosions for multiple opponents. Try to finish off the remaining units before the polymorphed enemy changes back. If you have a few precious seconds before you need to fight again, use it to either get away from the sheep so you have some distance, or if you desperately need the mana you can use evocation.
Now, it's a good idea to keep some potions with you at all times for emergencies. You will sometimes need that extra boost of mana or health. If you can, add some armor, intelligence, and spirit with a good combo of potions, scrolls, and other buffs. This will help you keep going without having to stop.
I hope this helped! I know there are other ways to go about your magery, and this might not be the best for you, but it's the best way for me so I thought I'd share in case it could help you.
Happy Hunting!
-FluffyBSFG