We need a better definition of "high level," not to mention how long it's going to be until you're actually expected to take on these guys.
Basically, you want to be a Cleric. Clerics can juice their CLs better than any Wizard ever, even without Master Spellthief (the feat I believe your DM is giving these Wizards). Check this out:
Level 13 Cleric:
Adept Spirit spell (Cleric 2, MoI): +1 CL
Orange Ioun Stone: +1 CL
Masterwork Holy Symbol: +1 CL (something that will boost a Necromancy [Death, Evil] spell)
Elder Giant Magic Feat: +3 CL
Agony: +2 CL
Necromancer Domain: +1 CL
Ankh of Ascention: +4 CL
Bead of Karma: +4 CL
Divine Spell Power Feat: +4 CL
Use this and DMM to cast a CL 34 Persistent Greater Consumptive Field in the morning, then walk through a hamlet, making your CL cap out at 51. That's a CL of 51 at level 13 (well, you have to kill 38 critters within the area in order to get the full bonus, but still, just walk into a densely-populated area). Your other persistent spells can also benefit from a number of these abilities again, pumping their CLs to 64, 68, or maybe even 70. If these spells are 10 min/level, you don't even need to bother with Persistent Spell, they're lasting over 10 hours already,
unextended. Oh, and you can even Persist a Spell Resistance spell for SR 80. When it's that ridiculously high, then it actually kinda matters.
Check out the resource here for even more potential boosts:
http://community.wizards.com/go/thread/view/75882/19872350/Raising_Caster_Level