Depends on the Save

There are very few ways to automatically ensure a target fails a save. There are more options for increasing the likelihood of failure. But you should be aware that there is no counter to Legendary Resistances short of forcing them to be burnt through. Save or die is pretty much non-existent in 5e and Legendary Resistance is there to reinforce that.

Strength & Dexterity Saves

Failure on either of these saves can be guaranteed by imposing any of the following conditions on the target: Paralyzed, Petrified, & Stunned.

Of those three, the most available to you as a Wizard is either Paralyzed or Petrified. Paralyzed is best accessible to you via the Hold Person and Hold Monster spells, both of which can be upcast to target additional creatures. As a 2nd or 5th level spell, respectively, you can attempt casting it multiple times in a day.

Flesh to Stone is the best means to access the Petrify effect, but as a 6th level spell it's probably much less useful to you. Furthermore, once you petrify the target it's really not necessary to do anything additional.

Constitution, Intelligence, Wisdom, and Charisma Saves

There is no way to force a creature to fail these saves besides NathanS's answer. But there are several things that you can do to set the stage with the team you have available (which I suspect is by design).

Your Grave Domain Cleric can impose a 1d4 penalty on an opponent's save by using the Bane spell. Given that success on a non-proficient save is often by only a point or two (even with advantage) this is big deal.

You or the party Warlock (if they took the appropriate invocation) can cast Bestow Curse on the target to force Disadvantage on saves with the preferred ability score.

Use Spells that Don't Allow Saves

Since you're in the realm of 6th level spells and up, there are a few spells you have available which don't grant saves at your disposal.

• Otto's Irresistible Dance
• Power Word Pain (which can be useful for setting up saving throw failures on anything but Constitution)

Depends on the Save

There are very few ways to automatically ensure a target fails a save. There are more options for increasing the likelihood of failure. But you should be aware that there is no counter to Legendary Resistances short of forcing them to be burnt through. Save or die is pretty much non-existent in 5e and Legendary Resistance is there to reinforce that.

Strength & Dexterity Saves

Failure on either of these saves can be guaranteed by imposing any of the following conditions on the target: Paralyzed, Petrified, & Stunned.

Of those three, the most available to you as a Wizard is either Paralyzed or Petrified. Paralyzed is best accessible to you via the Hold Person and Hold Monster spells, both of which can be upcast to target additional creatures. As a 2nd or 5th level spell, respectively, you can attempt casting it multiple times in a day.

Flesh to Stone is the best means to access the Petrify effect, but as a 6th level spell it's probably much less useful to you. Furthermore, once you petrify the target it's really not necessary to do anything additional.

Constitution, Intelligence, Wisdom, and Charisma Saves

There is no way to force a creature to fail these saves besides NathanS's answer. But there are several things that you can do to set the stage with the team you have available (which I suspect is by design).

Your Grave Domain Cleric can impose a 1d4 penalty on an opponent's save by using the Bane spell. Given that success on a non-proficient save is often by only a point or two (even with advantage) this is big deal.

You or the party Warlock (if they took the appropriate invocation) can cast Bestow Curse on the target to force Disadvantage on saves with the preferred ability score.

Use Spells That Don't Allow Saves

Since you're in the realm of 6th level spells and up, there are a few spells you have available which don't grant saves at your disposal.

• Otto's Irresistible Dance
• Power Word Pain (which can be useful for setting up saving throw failures on anything but Constitution)
• Reverse Gravity (this allows a save to keep yourself from flying up, but if it's centered on you and you don't have a fly speed you're probably going to be stuck where you're at)
• Maze
• Power Word Stun

Use Spells That Give You Lots of Chances (or rely on attack rolls)

It is way easier to up your odds of hitting the opponent by finding situations to give yourself advantage on attack rolls. Also, if one spell gives you multiple opportunities to inflict severe conditions on an enemy, then they're very useful. Consider the following:

• Eyebite
• Sunbeam
• Crown of Stars