A lot of this has been said, but what I did to level up my witch very quickly was I used the top spells allowed until she could upgrade objects magically. I sent her to the spa to get the $7,500 package (so when her magic depletes she can continue to cast spells without issue) and then I set up a line of upgrade-able objects (that can be upgraded in two ways, like the cheap dishwasher that you can upgrade to either Silent Running or Unbreakable, cheap computers, cheap TVs) and just sent her down the line of objects and back. I would have her upgrade them all to unbreakable then send her straight down the line to upgrade them to whatever the other upgrade was on each item. I just kept going back and forth up the line (only stopping to have her eat a conjured apple or take an invigorating elixir to keep her needs manageable). I found that leveled me up the quickest because there was no down time between "conjure apple">putting apple on ground>casting conversion ritual or finding someone to Fire Blast/Ice Blast.
Granted, if you get bored, hang around for the Full Moon and have at it with Zombies. I used to level my Witch that way - by having a set of zombie lawn ornaments from her freezing them all with the Ice Blast (this is also a good way to get your magic up to Upgrading things). I prefer upgrading though because the only element is your Sim with the objects (which won't move or disappear off your lot). When you get a line of objects going you can just queue them up and put it on the fastest speed. It's nice to hear how everyone else levels magic too!