How can I complete Bomb Defusal missions on Ironman Classic difficulty?

What is the most efficient way to do it on classic ironman? Lost about 7 of them, never won. Overwatch tactic just don't work since you should advance really fast. I'm most interested for tips for the first of those missions, when you don't have access to advanced equipment and perks.

The key, as you say, is advance really fast, but not recklessly. The power nodes are typically spaced out in such a way that you'll be able to deactivate one of them per turn as you advance to the bomb. If you can't, well there's a little leeway since you have a 2 turn buffer.

These aren't too hard since the enemies that show up are nearly always thin men and sometimes mutons (or muton elites late game).

Some general tips:

• Rely on support to disable the power nodes. They have +3 movement (assuming you get that skill), and they can use smoke grenade to cover themselves in times of trouble.
• If you have access to skeleton armor, I highly recommend it. The +3 movement bonus from that stacks with the support for some truly ridiculous movement ranges. Late game, ghost armor makes these missions trivial as you can just dash from node to node while cloaked.
• Keep 1 sniper back to provide long range cover support.
• Assaults are also great for this mission with their run and gun skill.
• Don't get into a extended fire fight with aliens, that's what they want you to do. If you have any explosives, use it to try to blow up their cover (especially if they are hugging a car).
• Make liberal use of hunker down rather than taking a pot shot at an alien at 30% hit chance.
• The bomb is almost always in the opposite edge of the map, and there's usually 2 'paths' to get there if you follow the line of power nodes. I don't recommend following both as that'll expose more enemies, but on the flip side, you get a little more leeway and the chance to flank said enemies.

I'd discourage using heavy guys. The bomb missions tend to have weaker units, and heavies often have low ammo. The only thing worthwhile about them are long range explosives.

Assaults + run and gun are key. I would also recommend liberal use of supports, mainly to cover your assaults with smoke when they get too far ahead.

Easiest way is to ghost and run. 2 supports with that can clear the bomb in less than 3 turns.

I almost never even bother to hit the nodes. After you clear it, you should still have 1 more ghost use. RUN your ass back and then take your sweet sweet time in killing the enemies.