What makes Kensai/Mage so OP?
Kensai are glass cannon fighters that can't wear armor but get absurd bonuses to attack and damage. Since mages already can't wear armor and cast spells as is, none of a Kensai's downsides actually negatively influence your mage class. Additionally the mage has access to powerful defensive spells that more than make up for the Kensai's lack of armor. So what you're left with is a character that rips apart everything they touch in melee while also having access to defensive magic that makes them virtually untouchable and offensive magic in case their melee attacking for some reason wasn't good enough.
The downside to the Kensai/Mage, and the reason I'm going to recommend against it for the OP, is that if you're starting out from BG1, you're committing to a frustrating experience playing through that entire game with a very squishy MC who can't used ranged weapons and will be prone to die very easily. It's not till BG2 your character will actually become an unstoppable engine of destruction.
OP: I'd advise going Berserker/Mage if you're not interested in struggling for half your playtime of the series. Berserker/Cleric is also very strong. As mentioned before, Ranger/Cleric takes advantage of a weird quirk that allows them access to Druid spells on their Cleric spell list which makes it a very potent combination as well. Another interesting option is Fighter mixed with Thief which will basically give you a free enemy one shot once per fight + another time for every invisibility potion you have.
Also you may want to consider multiclasses. A multiclass Fighter-Mage is not significantly less powerful than a dual classed one and won't struggle so much at early levels. Multi Fighter/Cleric and Fighter/Thief are also super good. A multi-class Cleric-Mage is possibly the most versatile character in the game and not only will you have access to an absurd amount of spells, mixing mage contingency and cleric spells can make for really interesting results.
