Firstly, I dug down for the two green crystals as I'm sure you did too. To be precise, I built two shields coming off the corner of the ground near the right green crystal, diagonally up-left (if that made sense).
Once collected, I immediately (ASAP) started building one blaster. I also started digging deep on the left side. towards the red crystal but only through my dig route! (so I wouldn't face creeper down another channel).
After you have the red crystal you should have dug most of the route out. Start building reactors. This is the hardest part in my opinion. The blaster should have been set to fire at drones only and by now the creeper will be starting to cover it. just after it has destroyed a drone, try moving it as deep into the cave as possible. Build as many reactors as you can until you have to build phantoms. (I think I started building the phantoms 5 seconds before the wave, because it takes a while for them to reach you.) You will need three phantoms in good positions, so make sure your reactors and the ship are protected.
After the first wave, deactivate the phantoms, and build as many reactors as possible. You should then start building one repulsor (the top one) which will hold the creeper for a while. You may need to set the blaster to fire at creeper too if things become tight.
Eventually build the second repulsor and then you are safe!


The above probably holds up for about an hour... It takes forever for the creeper to snake all the way into the cave. I don't think that setting up a dark beam is the best way to go given the time you have (saps a lot of energy and slows growth more than conversion bombs), but just to prove it is possible:

I am not convinced you need to dig that far, or that you even have to dig for that matter! Feel free to try your own thing.

As final general advice, digging horizontally for the ship is much cheaper than digging vertically. The more corners the creeper has to move around the more time you will have to set up a defence (a LOT more per corner). Two repulsors costs 60 less than 5 (20 ground blocks), and digging further buys much more time so it can, very rarely, be worth it.
Sorry for the extremely long spoiler!