To guarantee it is not dispelled,
Globe of Invulnerability
Any spell of 5th level or lower cast from outside the
barrier can't affect creatures or objects within it, even
if the spell is cast using a higher level spell slot.
You might need to coordinate with another caster if your buff also requires concentration. However, this Globe will protect your ally from getting dispelled, regardless of how strong the Dispel Magic cast by your enemy is. Seconded by Sage Advice:
- Can dispel magic end globe of invulnerability?
- Yes, dispel
magic can dispel the barrier created by globe of invulnerability,
but not any magical effects that are active inside
- Since the sphere has a range of self, the buffed person must either be able to cast the sphere, or stand near someone who can.
- While enemies can dispel the Globe, you can keep casting it and protecting the buff.
- Also, a Wish spell to dispel your buff will bypass the Globe. But it should be a good trade-off for you if the enemies do use a Wish just for that buff.
Other ways of helping, in addition to other answers (upcasting your buff, controlling enemies, counterspelling their dispels, or hiding your buffs), are based on decreasing the skill check made by the enemy. A Bard's Cutting Words, for example, is excellent!
When a creature that you can see within 60 feet of you makes an Attack roll, an ability check, or a damage roll, you can use your reaction to expend one of your uses of Bardic Inspiration, rolling a Bardic Inspiration die and subtracting the number rolled from the creature’s roll.
Other options are the Wizard's Portent feature:
You can replace any attack roll, saving throw, or ability check made by you or a creature that you can see with one of these foretelling rolls.
Or even other spells, like Bestow Curse:
Choose one ability score. While cursed, the target has disadvantage on Ability Checks and Saving Throws made with that ability score.
A final suggestion is to provide cover for your buffed ally. If some wall, or barrier is between both, the effect can't be dispelled, as spells require a clear path.