As far as manipulating the laws of physics go, the world of Diablo has an interesting take on it. The magical knowledge of the Diablo universe holds that magic spells are in fact ruptures in normal space-time where the mortal plane and the ether, which obeys different laws of physics than the material world, collide. These rifts take various forms for different types of magic.
About Janus not being able to compare to Lavos, that strikes me as odd. It's perfectly possible, though difficult, to beat Lavos with Crono alone, and his magic attacks aren't nearly as strong as Janus' repertoire. However, I suppose almost any character at level 99 could do it. Perhaps the reason one needs a Chrono Crew for the main storyline as opposed to a singular hero of time in New Game+ is because all the genes necessary for victory over Lavos caused some kind of a conflict in human physiology when put together. Realizing this, the planet would move to create time gates to unite the potentially most powerful humans ever.