The magic item Bag o Beans gives permanent ability bonuses about 10% of the time – specifically with this (random) result:
A nest of 1d4 + 3 eggs springs up. Any creature that eats an egg must make a DC 20 Constitution saving throw. On a successful save, a creature permanently increases its lowest ability score by 1, randomly choosing among equally low scores. On a failed save, the creature takes 10d6 force damage from an internal magical explosion.
The point? One could gain up to 30 in any / all scores pending enough successful Con-saves. That said, simple & easy constitution saving throws are hard to come by. Given vast time, energy, money &/or magical resources (dragons, vampires, elf lords, etc.), how would they prepare? The 35 damage (approx.) is not the issue – a failed save appears to give no ability bonus.
Basic Question: How can one improve &/or guarantee successful constitution saving throws given infinite time, money, support &/or magic to prepare?
All tricks are welcome / hook or crook: be that by advantage, use of luck, rare feat, class or race skill, potent-portent class features, advance preparation, stacking spells, phase of the moon &/or even advice from a ‘Heim’ Lich . Even the stress wish counts. Anything.
Note / FYI: This is of great interest to ültrapowerful creatures that have one ‘flaw’ ability. Take most ancient dragons (lacking dexterity), Beholders (low strength and dexterity), giants (often low dex) – or even some demon lords (Yeenoghu is a bit stupid, relatively speaking).