I dissagree about your comment
a high mineral income is KEY to the last lvl.
A high mineral income is not key for the end at least. In the last five minutes you should not have any more than 2-3 miners active, and those should be up in your base. The best strategy is to, instead of building upgraded miners, build a crapload of basic miners and have them mine a single or two close-together asteroids before scrapping them (it takes some up-front cost, but it's free in the long run) expand your base to as much of the asteroid field as you can (for this strategy you MUST have quite a few power stations) qith NO lasers, except MAYBE for a few basic ones to defend against small incursions (or a whole lot of them for large fleets) and a VERY well defended core. This core should have at least two rings of lasers, one (inner, only a half-ring) THEL++ and one pulser++.
so, with this core, which, as most everyone knows, the enemy will orient themseleves on and get destroyed by, you don't need massive amounts of lasers defending your mining operation, because the enemy never reaches it. You pull back on the side they jump in on and then they get torn to shreds by your core.
NOTE: you may want 2-3 missile launchers (fully upgraded) in the middle of your core to deal with swarmers (have them siwtched off to conserve minerals as you won't be generating any for the last 5 minutes of the last level, because of the constant bombardment)
so, the quick version:
-Have a well defended core with 5 solar stations and a storage tank, two rings of lasers and maybe a few missile launchers
-don't defend it, move it!
-turn those launchers off if you don't need them!
-the basic form of ANYTHING is free (yes you can have 50 repair stations for a few minutes and then switch them to 150 lasers, at NO COST)
-power relays can be used as decoys in a pinch, but you NEED to set them up early, and they don't have to be built (those 50 ring ships on last level? raped by just a few thels and about 1000 minerals worth of power relays...) Though, I said last ditch because it is EXPENSIVE and you can't use this tactic if you either don't have any money left or the enemy is too close.
-only upgrade lasers
so, that was rather long... I hope it helps... I haven't beaten the final level, but I got to about 1:00 left, and I difintiely could have survived if I had used my startegy, wich I of course thought of while reading the game over screen. also, as cerealkiller pointed out, having two fully upgraded THELs could easily kill mother ships, but they take up way too much energy and can't be turned off.