Omega Strike

These achievements require you to collect all chests and all Life Cubes in the game. Note that there is a third collectibles, Medkits, which is also displayed on the map screens, but they aren't needed for any achievements. Not only that, but they aren't even very effective late game, so I don't recommend going out of your way to get them. 

As for the chests and Life Cubes, there are essentially three ways they are located/found:
 

• At the end of side rooms on the map, with slightly more challenging platforming or enemies, and just sitting at the end for you to pick up
• "Locked" behind obvious abilities you need, such as behind rocks you need to destroy or across a gap or high up where you need double jump or dash
• Hidden behind false walls that you need to walk through to find the collectible just sitting there

You'll likely get all of the first two categories with relative ease, as they're all pretty obvious by just looking at your map for room exits and when you revisit each location with more abilities. The third catsgory is the trickier one. Some of them are pretty easy to spot thanks to obvious signs, but some are easy to miss. Thankfully (and surprisingly), the devs put together images of maps of each area with all collectibles marked on them. They can be found HERE. You can use them and your ingame maps to see which collectibles you're missing and how many. The story takes you to each location twice and, in general, the second visit is when you'll be able to access all collectibles, so take that opportunity to mop up areas while you're already there anyway, so you don't backtrack unnecessarily.