Got there eventually
Ended up using a slightly different early game strategy, instead of trying to get the path down early, i tried to maximise the effect the early towers would have, and upgraded them as soon as possible, and was able to fend off the first 3-4 waves with just 4 towers (iirc 2 red, 2 yellow guns) before being able to put down a temporal, and upgrade that ASAP so that those 4 towers were doing everything.
I think by the time the extended area appeared (w8 iirc) i had to dive on it just to stop cores getting to that wave, and it all came together in the end. It required a lot of selling and then putting down upgraded cannons as towers near the core were obsolete. Worked a treat.
I'll try Elite now, i think it'll be hard enough simply finishing it

you just seem to get a wave where theres 10-20 which are going strong and theres nothing you can do about it.
-- update --
Done it (Hard) another 2 times, just to double check and refine it a little. The last time, i didnt even need to use the extended area. 11 towers for the most part, with 3 cannons added just to make sure the crashers were dealt with.
Added image in the spoiler, the 2 green towers were mainly to block the path, might have been possible to just use boost blocks.
oh, and although the laser powerup was enabled, it wasnt used. Id put it on earlier to see if 'L' was the right button, and just kept selecting 'Play Again' after completing it, so its still available.
-- update 2 --
I cant do Elite, just get overrun too early on. Might be able to scrape runs with a core or 2, but doubt it.
I've gone back to Hard to try and get an improved run, and yet i cant beat my old score.
I've got more resources, ive built fewer towers, upgraded less, and ive sold none. Before this run, i did a decent attempt and it was looking at being around 91k, but cos i was going to clear it with ease, i skipped back a few waves and avoided doing all the upgrades. Less firepower = higher score etc... 90.4k. So this run was with 0 resets, thinking maybe they account for people constantly redoing waves. IMO its bordering on cheating as features go, same applies in other games where you can rewind and avoid screwing up. But its a feature, and apparently doing it without use doesnt have any substantial impact.