There's a way to win the “Closing the Gates” quest quite easily, even on Brutal difficulty.
Just rush. Seriously. That's the way to win. Place miners on the two first visible glyphs immediately, walk with 2 more miners to the south-east branch and place them on the glyphs, too. It should be safe. The go “all in” with your fighters to the north-west branch and meet goblins. If you are good, you might even defeat all of them, but if not, you could manage to distract the goblins in combat while sneaking two units towards the glyphs. Since you win the moment you have placed the last unit on the glyph, you don't have to defeat all goblins.
You have to be quick in Brutal difficulty, but it is clearly doable even there.
Suggestions:
I suggest to add another victory requirement: As soon as all glyphs are occupied, the players has to keep them occupied for a certain while. The story already quite neatly explains this with “the gates need time to close”. :-) When an unit dies on a glyph or walks away, the timer resets. This should defeat the whole rushing strategy.