Well, I went once over the top, twice through the steel and once down and up again.
First through the steel solution is bombing to make the steel thinner, setting a blocker as high as possible, wrong-way-mine and freeing the blocker doing this, and bashing through the rest of the steel.
Second solution is using a blocker to push the though the wall, as tseug did.
My going down way might be the intended one though:
Mining down and using a blocker to turn the miner around, that way you get through using only one miner.
Bashing through the pillar, and sending two climbers up at the left of the right half using builders. Mining and turning the miner around again. Using a bomber to get through to the miner tunnel. Then building to get to the exit.
This solution requires almost all skills, and has only about one minute left.