On saturday i playing with batman 75th batman beyond an wos deadpool and i used the free smoke cloud to make hindering front of them. I thought that as long as line of fire goes through hindering terrain that line of fire is blocked, because they are in stealth. I was told that the character has to occupy hindering terrain to be in stealth. Who was correct.

it only has to cross the hindering for grounded. giants and colossals must be in hindering.

It all depends where the Line of Fire is being drawn from. If it is from elevation higher, then they those characters have to be in hindering terrain. If on the same elevation then LoF has to be drawn through any square of hindering between or occupied by those characters.

And as stated above, if they were Giants or Colossal then the squares they occupy must be hindering no matter what the elevation LoF is from.