You are incredibly limited in options
We don't have access no non-Warlock spells, including glyph of warding which would help us get around concentration requirements. In fact, we have very very few options available at all. Almost nothing causes disadvantage on saving throws, though the Paralyzed, Petrified, Stunned, and Unconscious conditions all cause automatic failure of Strength and Dexterity saving throws, which is probably better than disadvantage:
The 6th level Unearthed Arcana psychic crush spell (Strength & Dexterity):
This comes from the "Fighter, Rogue, and Wizard" Unearthed Arcana (pdf link):
[...] The target must make an Intelligence saving throw. On a failed save, the target takes 12d6 psychic damage and is stunned for 1 minute [...]
The 7th level power word pain spell (Strength & Dexterity & Intelligence & Wisdom & Charisma):
This gives a target disadvantage on all saving throws except for Constitution saving throws; however, it only works on targets that have 100 hit points or less and that are not immune to being charmed.
The 8th level power word stun spell (Strength & Dexterity):
[...] If the target has 150 hit points or fewer, it is stunned. [...]
This has the same effect as psychic crush but it is not an Unearthed Arcana spell, though it also only effects those with 150 hit points or less.
And that's it, there are (currently) no other non-concentration Warlock spells that impose disadvantage on saving throws. Even the various potentially useful spells from Eldritch Invocations (hold monster, bane, bestow curse, and slow) would all require concentration and so cannot be used.
The 8th level feeblemind spell (Intelligence & Charisma):
[...] On a failed save, the creature's Intelligence and Charisma scores become 1. [...]
This does not technically give disadvantage on Intelligence and Charisma saves but it can have a stronger (or weaker) effect on them than disadvantage would.
The 5th level synaptic static spell (Constitution (Concentration only)):
This spell causes a creature's Constitution saves to maintain concentration suffer 1d6 as a penalty. This is very slightly better than disadvantage though it does not apply to Constitution saving throws in general.