#BG3 life hack: if the only character who can cast Aid is a Paladin, you can upcast it only as a LV3 spell so that you get a +10 to the hitpoint maximum. But if you make the Paladin drink a Supreme Elixir of Arcane Cultivation, they get a LV4 spell slot, even when they don't naturally have that, and you can then use that to upcast Aid as LV4 and get +15 to the hitpoint maximum. After you've done this, the Paladin can drink some other elixir if you need them on the field.
Of course if you have a Cleric in your party, they can upcast Aid as LV6 spell. But sometimes you just don't have a Cleric