To be fair,
Strange still in learning stage when attck on london sanctum begins..
He is still learning how to use an enchated weapon with mordo. (easter egg: staff of living tribunal - one of the powerful being in mcu)
Strange become powerful because he learned the time altering spell thats it..
He still dont learn anything else much making him still a half baked sorcerer.. Kira this movie just intro him only.. Maybe in avenger infinity war he become powerful as sorcerer supreme..
But kinda bummed when the eye of agamotto is an infinity stone.. That thing should be strange source of power.. Not just to access the time manipulating spell. And kantoi its a time stone because that time altering thing stuff..