I did this on Chilled difficulty on 50% speed with the Ovoid ship. I felt that 25% speed was too slow and took too long, especially after a random screw up in the later stages. On 50% speed, it took about 20-25 minutes to complete. It'd take twice as long on 25%.
I also experienced a problem several times where my game would crash and I'd lose some progress upon restart. It happened twice after the 5th stage (after Mecha Lilith) when I tapped the screen to continue to the next stage. The stage results screen comes up very shortly after killing Mecha Llilth and I was finally able to proceed without crashing by waiting for her death explosion to dissipate under the results screen before tapping the screen to continue.
I took the left path on every stage EXCEPT stage 2. On the right path for this stage, I flew to the right (with my ship staying around the center-left of the screen) and eventually encountered a 1-up pie, which brought me up to 30 ships, resulting in huge damage output (especially with shields).
I kept my ships clustered just big enough to produce the big blue laser, making it easier to dodge bullets.
Shields will instantly kill every non-boss enemy and will even one-shot a few of the bosses (stage 3 and 5 bosses IIRC).