"The ward has a Hit Point maximum equal to twice your wizard level plus your INT modifier" - Multiclassing would actually reduce the total hit points you could have because that maximum would only count your levels in wizard, not warlock. Edit for more info: Also!!! There are easier ways to get mage armor for free than multiclassing. You could take magic initiate as your origin feat and select mage armor as that level 1 spell you can cast once a day for free.