At 4th spell level, crushing despair is the go-to for area-effect, while bestow curse is the preferred single-target. Note that bestow curse can be applied to Will saves (or any save you want) twice, once with the general -4 to most d20 rolls and again with the -6 to a target stat.
Earlier than that, you can use mad hallucination. Ray of sickening deserves special mention for being even earlier at first level and targeting Fortitude instead of Will, it's nice to get you started. If you have a cleric in the party, doom is also first level.
The Enchanter wizard's Aura of Despair class ability (or the Enchanter-Manipulator wizard's Shape Emotions class ability) can debuff saves as well.
Don't forget you can also buff your spell DC rather than (or in addition to) debuffing their Will. Increasing your casting stat (Intelligence for Wizards) is the obvious way (permanently with items or levels or temporarily with spells like fox's cunning), but feats like Spell Focus (Enchantment) also make it harder to resist your magic.
If you're willing to consider other arcane classes, for my controllers I usually prefer Fey bloodline Sorcerers, who get +2 to their Compulsion save DCs. Kitsune sorcerers can also spend their favored class bonuses for a +1 to enchantment save DCs every 4 levels. These two bonuses can be used together. A friendly witch can use their Evil Eye or especially Misfortune hexes (this could be you, most debuffs and control spells are on the Witch list as well) to penalize Will saves. Arcanist is probably a poor choice for controllers, but they can boost their save DC a point using their arcane reservoir.