In my next campain my group will face a chokepoint with archers on top of the mountain and melee down where they need to pass exept if they want to climb to get up on the high ground. My question is can our wizard hit both the people on the ground and the ones on the top (50 feet high cliff) at the same time (with the same attack)
In canonical two-dimension combat, blasts and bursts have a square shape (approximating a round one). If the third dimension is relevant, remember that blasts and bursts are cubical (approximating a sphere).
So, an area burst 3 is a 7x7x7 cube. You have to aim the center of your spell (the origin square) 4 squares above ground in order to fill the entire volume of the cube.
50 feet equal 10 squares, so you need a really huge burst (at least an area burst 5 that produces a 11x11x11 cube). If you could accomplish this, and there are no obstructions from the center of the burst to your targets, you can hit both the ground-based melee and the archers on the cliff.