This game does not feature a quicksave mechanic nor does it have any indication of how many collectibles and secrets you have found or missed outside of the end level screen for each level on a single playthrough.

If you're a completionist, you may find it hard to keep track of what you missed in one playthrough, so this guide is meant to help those who want to get the following achievements: "Clear" (Kill everyone), "Explorer" (Find all secrets) and "Tremors" (Destroy all barrels).

There is one data core per level so I won't be including those - there's always just one (1).

1-1

• Enemies: 30
  
• Secrets: 1
  
• Barrels: 1

1-2

• Enemies: 52
  
• Secrets: 1
  
• Barrels: 3

1-3

• Enemies: 50
  
• Secrets: 1
  
• Barrels: 7

1-4

• Enemies: 48
  
• Secrets: 1
  
• Barrels: 13

1-5

• Enemies: 60
  
• Secrets: 1
  
• Barrels: 8

2-1

• Enemies: 55
  
• Secrets: 3
  
• Barrels: 10

2-2

• Enemies: 63
  
• Secrets: 2
  
• Barrels: 14

2-3

• Enemies: 84
  
• Secrets: 1
  
• Barrels: 15

2-4

• Enemies: 103
  
• Secrets: 1
  
• Barrels: 5

2-5

• Enemies: 95
  
• Secrets: 1
  
• Barrels: 11

3-1

• Enemies: 62
  
• Secrets: 2
  
• Barrels: 16

3-2

• Enemies: 64
  
• Secrets: 2
  
• Barrels: 10

3-3

• Enemies: 57
  
• Secrets: 1
  
• Barrels: 20

3-4

• Enemies: 80
  
• Secrets: 1
  
• Barrels: 12

3-5

• Enemies: 1
  
• Secrets: 0
  
• Barrels: 1

Zockerx96 was kind enough to post these secrets or tips for finding some of them:

-level 1-1, 1-2, 1-3, 1-4, 1-5, 2-1: Secrets are behind optional key doors (like in level 1-1, just collect the key and then backtrack) or behind walls that you can destroy by shooting a barrel next to it, btw. always shoot all the barrels you see.

-level 2-2: Collect the RPG, then you must find 2 walls with a lot of cracks on it, one is at a dead end near a laser, the other one is almost at the end of the level at the right of a door mit two flags. Shoot it with the RPG to open it.

-level 2-3: One destroyable wall at a dead end near the end of the level.

-level 2-4: One destroyable wall near the start of the level, in the right corridor.

-level 2-5: One destroyable wall near the start of the level, in the right corridor, AND two other destroyable walls behind that one.

-level 3-1: From now on there are fake walls with a head on it. So both are near the start, one is in the second right corridor, (a melee guy will come out of the wall) and one in the second left corridor.

-level 3-2: A destroyable wall is in the "old-wall-area", a laser and some RPG ammo will be next to it. Also there is a fake wall near the end of the level next to some HP items.

-level 3-3: A fake wall is in the "box-maze" AND behind that one is a destroyable wall.

-level 3-4: A fake wall is quite near the end, to left of the 3 key doors AND behind that one are 5 destroyable walls, so save enough RPG ammo for that secret.

level 3-5: There is no secret, the data core can't be missed.

That's all!

In total, there are

15 Data cores
904 Enemies
19 Secrets
146 Barrels

Thank you for reading this guide. If you found it useful, hit the thumbs up.

I might eventually make a full cores/secrets/barrels guide with screenshots, but I'm not promising anything.