The MC really can't go with an acrual steam engine concept. Though it would probably complicate the tunnel, requiring vents and ventillation ducts for the smoke and steam.
Instead of that he should set up a large land to pop under the dragon imprison it with a couple of dozen giga-golems to hold it down quickly and give the smack-down.
JacobS: @Armand 2176 if that's the case then mini fence and its over (height around 5cm)
I'm not sure but I think he needs to erect walls around his plot of land to project that invisible cloak or the only thing that's going to disappear is that land.
Fizy: This idiot totally forgot that he has the conceal garden ability.
MC needs to get his priorities straight. I mean there is niaive but this is bordering ret*** level. That idiot already attacked them twice in this trip and you would think he was an innocent villager....
