Sometimes I have played games where people use very cheesy strategies with mages, like keep them hidden behind a cliff, bring there and back to cast spells and hide again... I am not super competitive but this feels counterintuitive considering that charging must see the target before moving, so why not also mage?

So the question is, do mages need to see their target before moving, or only during the movemeng phase, to cast a spell on the target? I havent found any explicit rule about this.

I don't think that you need to have line of sight to a model at the start of your move to be able to cast a spell (like sorcerous blast) on it.

The rulebook explicitly says that to move into combat a model "must have line of sight to its intended target before it starts its move".

But for spell-casting there's no such mention, just that the caster must be able to see their target and that they can use the power "before they move, during the move or at the end of it".

If you have Gandalf or Saruman hiding behind scenery then jumping out and blasting heroes off their horses each turn, this could be quite annoying. But they're a huge points cost, so I don't think it's too unfair ... they have to make their points back somehow, and against some armies probably won't ... eg. blasting one orc or goblin per turn in a horde army won't slow it down much.