I have yet to get this, but I've had a bit of time working on this one to give a good overview.
The Limp Berserker is one of the unlock-able ships. You first need to collect the 5 ship pieces for it. This collecting is completely random unfortunately so when you get it will be tough to estimate.
The Limp Berserker has a few differences than your classic fighter:
1) Maximum Fire Rate
2) Insanely low health
Change 1 gives you a high damage output. What this really does is guarantee that your gun is fully upgraded at the start of the level. I have found this is really nice to have and has assisted in me getting a few new medals I was struggling with.
In reality, change 2 really is a one hit kill health bar. You will need to be really good at dodging and likely have a good stock of powerups to survive some levels. Here are some additional tips:
- The mega bomb not only kills many enemies, it also clears out attacks within the blast radius. Some of the more hectic moments can become a lot better with this change.
- Having the lucky shield card can save you for one hit. It appears to be a bit random for activation though, so don't count on it saving you.
Keep in mind, for this achievement you only need to beat all 13 of the base levels (not B1 and B2). Additionally, beating them on Normal is sufficient. Additionally, the medals you earn doesn't matter either. All that matters is that you go from the beginning of all 13 levels to the end and see the "Mission Clear" message.
You can check your progress by going to the "Collected Parts" portion of the main menu. Any ship you have all the parts for will show you which missions you have completed at this point for each of them.