Fafnir's Swamp, located right of Wen (through Zender's Woods) or left from Silverbrook (through Devon Forest - the Wen route is shorter) is a good place for experience, but bad for treasure. What you'll be hunting are Slimes, Sludges, Goo, Gunk, Muck, and Mire, hereafter referred to as "goo". Dragons bite too hard for a wizzie to safely solo, and the various Souls (the floating skulls) give no better experience or treasure than goo, at a much higher risk.
Before hunting the swamp, you will want to get your alignment to Malevolent or lower, for two reasons. First, the Souls in the swamp are evil, and will jump you repeatedly if you're neutral or good, and second, the 90% poison immunity from running Dark Gauntlet and praying at the altar for Duach's Blessing helps immensely.
You can start with single goo at around level 150 or so. Green goo (Mucks and Mires) are the lowest level, blue goo (Goos and Gunks) are in the middle, and brown goo (Sludges and Slimes) are the highest level, and therefore the hardest. Start with a single green one if you're low level and/or have never fought goo before, to get an idea of what you're facing. You will want shift potions if you don't have a Hood of Shifting, and a few regen potions aren't a bad idea if you don't have the Enid's Blessing spell. Cast Greater Invisibility, invul yourself, drink a shift (if needed), and cast extension. Start the fight without casting Defenselessness on the goo or cursing it, goo has very low AR, so it's not needed. If you're under level 200, summon a Daemon first round, placing it on the far right side of the screen. (Daemons are much better than Faery Queens where goo is concerned.) Placing it on the far side of the screen will get the goo away from you if your hold fails, and make it waste some of its movement rate. Second round cast Hold. Goo is approximately 33% MR, so sometimes it takes a couple of tries. Once the goo is held, summon more Daemons, then cast Duach's Vengeance (if you have it) or Mass Drain or Lightning Bolt (if you don't) until it's dead. You can also throw your dagger, if you prefer, just keep a few Daemons out at all times. Stay invisible and stay shifting.
Groups of 3 or less work better held. Groups of 4, and especially groups of 6 can be zerked. This is where the real experience bonanza comes in, a group of 6 will give you 10k points even at level 500. Don't try zerking the big groups solo until you've got some hit points (level 350 or so for an AIEW), and do use Mass Drain to make up the damage you'll take from the occasional zerked hit. Groups are excellent levels for a wizzie/BC'er combo, as well, just make sure you keep your hitter healed and invulled. The down side to goo hunting is that it's a gold losing proposition. The drops are terrible, and will not pay for your mana or repairs.