This guide doesn't contain any spoilers, except for the Levels section, which has previews from level selection screen.

None of the achievements are missable.
You can always go back to previous levels and replay everything as much as you wish.

The most important thing to look out for is secret collectibles (fruit, NOT gems). Even though you can go back to find any fruit you missed, you will not know when and where you missed them, so even if you have all but 1 fruit, you need to replay every level where you haven't found a fruit yet, without guarantees to find it. There is, however, a complete guide for the locations of fruit, which makes it easy to see which levels still have fruit and saves you from a ton of potential frustration.

Most of the levels closer to the end are very difficult (in a frustrating way), so if you're planning to get 100% but not sure about your skills (or persistence), you may want to check out the rightmost available consoles sooner rather than later.

The recently added "Hardcore" achievements are also very difficult, and you should be aware of them if you don't want to replay 40 levels later. The hardest level to do them in is probably ZX SPECTRUM: 6. MONSTER PALACE, so you should try it before getting too committed.

• 
  Spring Loaded

  500 Jumps
  Jump 500 times in total. You'll eventually get it. Can be checked on Stats screen.
  
• 
  Beam Me Up

  First teleport use
  Use any teleporter. You'll eventually get it.
  
• 
  Profesor Pixel Pal

  You have just met professor pixel for the 1st time
  When you see the blue square old guy, use the action key to talk to him.
  
• 
  Determined

  Retry 20 times in a row on the same level
  Die 20 times in a row on one level.
  
• 
  Arcade Champ

  total play time over 2 hours
  Play for a total of 2 hours. Only time spent playing some levels counts. Can be checked on Stats screen. If the stat keeps resetting for some reason, you will have to play for 2 hours without exiting the game.
  
• 
  Noob

  500 Deaths
  Die 500 times in total. Can be checked on Stats screen.
  
  
• 
  Cocktail

  collect 10 fruit
• 
  Salad

  collect 25 fruit
• 
  Super Juice Legend

  Collect all fruit
  The secret specials (fruit) are what you should pay attention to the most, because the game doesn't let you know which levels have fruit.
  If you got a fruit, you don't need to finish the level (unless you're going for the hardcore achievements); you have it permanently, and it disappears from the level permanently (unless you enable "respawn collected for hardcore" in options, which you should do in most cases).
  Levels in which you got a fruit are indicated on level selection screen. You can see your total in the special stats screen.
  Detailed information:
  http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=335058200
  
  
• 
  ZX81 Appreciation

  Achieved for collecting all zx81 special gems
• 
  A2600 Appreciation

  Achieved for collecting all A2600 special gems
• 
  ZxSpectrum Appreciation

  Achieved for collecting all ZxSpectrum special gems
• 
  BBC Appreciation

  Achieved for collecting all BBC special gems
• 
  CPC464 Appreciation

  Achieved for collecting all CPC464 special gems
• 
  Commodore 64 Appreciation

  Achieved for collecting all Commodore 64 special gems
• 
  GameBoy Appreciation

  Achieved for collecting all GameBoy special gems
• 
  NES Appreciation

  Achieved for collecting all NES special gems
• 
  APPLEII Appreciation

  Achieved for collecting all AppleII special gems
  Get all the special gems in the corresponding machine (set of levels).
  Only the consoles in this list have gems, and there are exactly 1 in each of their levels (8 per console). If you got a gem, you don't need to finish the level (unless you're going for the hardcore achievements); you have it permanently, and it disappears from the level permanently (unless you enable "respawn collected for hardcore" in options, which you should do in most cases).
  Levels in which you got a gem are indicated on level selection screen. You can see your total in-game in the bottomright corner.
  In most cases finding the special gems is easier than completing the level, and you can easily go back in case you missed some, so don't worry too much about these.
  
  
• 
  Hardcore

  For 10 Levels, Collect Fruit, Special Pickup and complete level in the same attempt.
• 
  Super Hardcore

  For 20 Levels, Collect Fruit, Special Pickup and complete level in the same attempt
• 
  Mega Hardcore

  For 40 Levels, Collect Fruit, Special Pickup and complete level in the same attempt
  There are exactly 40 levels that have both a secret gem and a fruit. So you need to get all the normal gems, the secret gem, the fruit and reach the exit without dying, in each of them. Levels in which you've done this are indicated with a green "cool" face on level selection screen.
  You must enable the setting "respawn collected for hardcore", because otherwise the fruit and gems will just disappear after they have been collected once.
  
  
• 
  Pixel Legend

  Achieved for completing all levels
  Complete every single level in all the machines. If a level is completed, you can see your best time under its picture in level selection screen. Whether you've completed a level is not related to the gem indicators.
  This achievement is the sum of 2 following achievements:
  
• 
  8 Bit Hero

  Achieved for completing all 8-bit machines
  The 8-bit machines are:
  
  • ZX81
    
  • ATARI 2600
    
  • ZX SPECTRUM
    
  • BBC MICRO
    
  • AMSTRAD CPC464
    
  • COMMODORE 64
    
  • GAME BOY
    
  • NES
    
  • APPLE II (unlocked by completing every 8-bit console prior to it)
    
  • MASTER SYSTEM (unlocked by collecting all special gems)
• 
  16 Bit Hero

  Achieved for completing all 16-bit machines
  The 16-bit machines are:
  
  • SNES
    
  • AMIGA
    
  • MEGA DRIVE (unlocked by collecting all fruit)