Sky Force Reloaded

The Gladius 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 Gladius has a few differences than your classic fighter:

1) Increased damage

2) Reduced attack range

This fighter is high risk/high reward. The short range means you need to get in close. On casual, most enemies will die extremely quick with this fighter coming at it (especially when upgraded). The main issue becomes being able to get in that close. Some enemies have fire rates that envelop them at the beginning of their attack.

Because of these issues, I'd recommend having an extra shield or two when going through levels. Another possibility (that I didn't test) would be the technician that slows enemy bullets. This might help with getting the attacks.

I will say that this fighter really worked well for other tasks. I've knocked out some medals that had been giving me trouble. Namely, the medals for killing 100% of the enemies were more possible as a result. In particular, I got 100% kill on Level 13 without needing powerups (although a megabomb helped towards the end with clearing the spinning blades that split into energy balls).

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.