I'll cover things again, so as to try to get a single, complete list, and because I think there are some additional points to mention. I make the split into two categories:

Buffing your own save DC:

• Improving your wisdom score up to 20:

• Ability score improvements

• Feats: Resilient, Observant, Elven Accuracy (XGtE; Elves, half-elves)

• Ioun stone of Insight

• Improving your Wisdom score beyond 20:

• Book of Exhalted Deeds (upto 24)

• Tome of Understanding (up to the innate max of 30)

• Deck of Many Things: Star (upto 24) (pull cards from the deck at your own risk)

• Increasing your proficiency bonus

• Ioun stone of Mastery (+1)

Weakening the target's constitution save:

• Negative modifier

• Bane (1d4)

• Wild Magic Sorcerer's Bend Luck (1d4)

• Disadvantage:

• Bestow Curse

• Sickening Radiance (through granting exhaustion)

• Staff of Withering

• Claw of the Wyrm Rune's Wyrm Ward (Storm Kings Thunder; Consumes the item, requires 8h prep work on the area, and only works on dragons)

• Replacing the roll through Wizard School of Divination's Portent

I'll cover things again, so as to try to get a single, complete list, and because I think there are some additional points to mention. I make the split into two categories:

Buffing your own save DC:

• Improving your wisdom score up to 20:

  • Ability score improvements

  • Feats: Resilient, Observant, Elven Accuracy (XGtE; Elves, half-elves)

  • Ioun stone of Insight

• Improving your Wisdom score beyond 20:

  • Book of Exhalted Deeds (upto 24)

  • Tome of Understanding (up to the innate max of 30)

  • Deck of Many Things: Star (upto 24) (pull cards from the deck at your own risk)

• Increasing your proficiency bonus

  • Ioun stone of Mastery (+1)

Weakening the target's constitution save:

• Negative modifier

  • Bane (1d4)

  • Wild Magic Sorcerer's Bend Luck (1d4)

  • Mind Sliver (1d4; TCoE), thanks to Jesse Quimby for pointing this out

• Disadvantage:

  • Bestow Curse

  • Sickening Radiance (through granting exhaustion)

  • Staff of Withering

  • Claw of the Wyrm Rune's Wyrm Ward (Storm Kings Thunder; Consumes the item, requires 8h prep work on the area, and only works on dragons)

• Replacing the roll through Wizard School of Divination's Portent

I'll cover things again, so as to try to get a single, complete list, and because I think there are some additional points to mention. I make the split into two categories:

Buffing your own save DC:

• Improving your wisdom score up to 20:

• Ability score improvements

• Feats: Resilient, Observant, Prodigy (XGtE; human, half-orc, half-elves), Elven Accuracy (XGtE; Elves, half-elves)

• Ioun stone of Insight

• Improving your Wisdom score beyond 20:

• Book of Exhalted Deeds (upto 24)

• Tome of Understanding (up to the innate max of 30)

• Deck of Many Things: Star (upto 24) (pull cards from the deck at your own risk)

• Increasing your proficiency bonus

• Ioun stone of Mastery (+1)

Weakening the target's constitution save:

• Negative modifier

• Bane (1d4)

• Wild Magic Sorcerer's Bend Luck (1d4)

• Disadvantage:

• Bestow Curse

• Sickening Radiance (through granting exhaustion)

• Staff of Withering

• Claw of the Wyrm Rune's Wyrm Ward (Storm Kings Thunder; Consumes the item, requires 8h prep work on the area, and only works on dragons)

• Replacing the roll through Wizard School of Divination's Portent

I'll cover things again, so as to try to get a single, complete list, and because I think there are some additional points to mention. I make the split into two categories:

Buffing your own save DC:

• Improving your wisdom score up to 20:

• Ability score improvements

• Feats: Resilient, Observant, Elven Accuracy (XGtE; Elves, half-elves)

• Ioun stone of Insight

• Improving your Wisdom score beyond 20:

• Book of Exhalted Deeds (upto 24)

• Tome of Understanding (up to the innate max of 30)

• Deck of Many Things: Star (upto 24) (pull cards from the deck at your own risk)

• Increasing your proficiency bonus

• Ioun stone of Mastery (+1)

Weakening the target's constitution save:

• Negative modifier

• Bane (1d4)

• Wild Magic Sorcerer's Bend Luck (1d4)

• Disadvantage:

• Bestow Curse

• Sickening Radiance (through granting exhaustion)

• Staff of Withering

• Claw of the Wyrm Rune's Wyrm Ward (Storm Kings Thunder; Consumes the item, requires 8h prep work on the area, and only works on dragons)

• Replacing the roll through Wizard School of Divination's Portent