How to protect my party against level drain in BG2 i am in watchers keep

trinkets only IIRC

According to Wiki: Negative Plane Protection, you can get this protection from a 4th-level priest spell. However, it is single-target, and short duration, which limits its utility. That page also lists some items that confer the protection for as long as the item is equipped. Certain characters are immune to Level Drain, such as the Undead Hunter (Paladin kit).

According to Wiki: Status Effects, you can also block the level drain effect with Barbarian Rage or with Enrage. These are also short duration effects. Lesser Restoration can reverse the effects of a level drain.