I suck at SHMUPS. Horribly so. But having just popped this one I thought I would pass on a few tips that I hope everyone finds helpful.
1. Like any achievement that has no shortcuts you just need practice. If you don't make it, just keep trying until you establish a solid rhythm and strategy;
2. I suggest playing in Easy mode, I found the higher-tiered difficulties too challenging and keeping a solid pace you should still be able to maintain a 3x multiplier;
2. The general strategy I used was to focus on the parasites to ensure the guardian has appeared & been destroyed before your 3x multiplier drops back down;
3. If you see a parasite, kill it then follow the direction it was heading as I found the parasites tend to bunch together. The last thing you want is to take too long and the Hunters turn up, in which case you are screwed;
4. When fighting Guardians I always started backtracking and shooting with cannons then once they get very close hit them with a bomb then fly and continually shoot in circles. I never used any other weapons other than cannon and bombs. This may or may not work for you so find the best strategy you are comfortable with for destroying the Guardians;
5. Once you reach Wave 20 you have access to Overdrive which gives you 2 consecutive bombs making this a little easier.
Unfortunately I don't have a vid capture set up, but for some visual footage of Arcade mode please see Youtube video from PickHutHG below.
Obtaining this achievement will also net you:
AND help with numbers for grinding for what will be the final achievement of the game:
Best of luck to all and if anyone finds a Youtube vid of this achievement or has any constructive criticism to improve this solution, please let me know so I can improve it.