Easy "Better Together" Achieve...
	Easy "Fat Stacks" Achievement...
	Easy "Masteralladon" Achieveme...
	Easy "Rage Master" Achievement...
	Various Steam Achievements...

All that's required to gain this achievement is to beat the last level again, the asteroid level, with a co-op player. You can simulate a gamepad if you don't have one, using a combination of vjoy to create a virtual gamepad and x360ce to make it an xbox controller. If you map the virtual gamepad to the same keys you control rex with in the game, the two players will more or less stay together in the level, making it a lot easier to control. However the level doesn't fail if one player dies, so you can also use different controls for both players and just have any other person help you out, or swap between the two, using the 5 second invulnerability on any player that dies to get them to a safe place before swapping to the other. You also don't have to get the good ending. Just get to the bomb room and then wait out the timer and get the 'bad' ending, that will be enough.

Play the "Scrollway" level and enter the hidden bonus level. It can be found after the laser section. Drop through the hole in the floor and stand on the glowing platform. After you are in the bonus level, fly through the first row of coins to increase your multiplier to 4x then reach the end of the level all the way to the right and collect the ruby. Note: Watch your multiplier, as it decreases rapidly.

Despite what the other guide says regarding 'mastering' a level, in order to master a level you not only have to get all three stars and any treasure, but you also need to collect all coins in the main level as well, (so that doesn't include coins in the bonus stages, but you will still need to get the treasure boxes in those stages on a few of the levels). It's easy to tell which levels you've mastered when viewing the map, as they will then say "100% complete" and the frame showing your best time and stars will change to a fancier gold border. There is a very comprehensive guide on Steam which tells you where all the coins are on every level.

This is a lot easier than it sounds. If you are playing ragequit rex, start in easy, which lets you take a hit before losing a life. Then if you do get hit, restart the level by hitting 'select' on you gamepad or the equivalent keyboard function. This will instantly restart you at the beginning of the level and you won't lose a life. It's still tough, but if you have already beaten the 3 star challenges on all levels, you should be familiar enough with the levels to be able to finishing them without getting hit.

Complete the indicated task to unlock the achievement.

Achievement	How to unlock
3 Pointer	Score 3 points in Holoball in the Rexercise Room.
Backlogasaurus	Play the game instead of letting it go extinct in your backlog.
Basic Training	Complete Boot Camp.
Beat the Game	Complete adventure mode on normal.
Beat the Game (Properly)	Live to fly another day.
Better Together	Complete adventure mode in co-op.
Big Money	Spend 1,000,000 gold in the shop.
Brobot	Defeat the second boss.
Chunky	Defeat the Yeti using only snow chunks.
Coinosaur	Collect every coin in a level.
Color Blast	Buy 30 different jet stream colors for Rex.
Combonaut	Have a 4x coin combo multiplier lasting over 30 seconds.
Dinonaut Master	Beat Adventure mode on normal, getting every star and hidden package, under 3 hours.
Dribble	Get a 5x bounce combo on the first boss.
Fat Stacks	With a 4x combo collect a ruby.
Filthy Casual	Complete adventure mode on easy.
Frontin'	Use death to your advantage in Cold Front.
Happy Family	Beat a level in Co-op mode.
Headhunter	Buy 25 different heads for Rex.
JumpJet Rekt	Die 500 times.
Master Fighter	Get 3 stars on all 4 bosses.
Masteradon	Master one level.
Masteralladon	Master all levels.
NegaRekt	Defeat the fourth boss.
No Jumper	Beat the target time for a level without using Jump.
Nothin' Special	Beat the target time for a level without using Dash, Drop or Rocket.
OCDinosaur	Collect every coin in every level.
Planted	Defeat the third boss.
Pompeii	Cover a level in ash.
Quick Shooting	Kill the Big Globowski in under 30 seconds.
Rage Master	Beat Ragequit Rex without losing any lives.
Ragequit Rex	Complete the RageQuit Rex mode.
Rainbow Rex	Buy 50 different skin colors for Rex.
Rex In Peace	Pay the ultimate price.
RexBox	Collect all the hidden items.
Scrollcut	Use the shortcut for a photo finish in Scrollway.
Seed Slam	Find Seedmours weak spot!
Shoe Shopping	Buy 20 pairs of jumpboots for Rex.
Shopaholic	Spend 100,000 gold in the shop.
Skillshot	Use your projectile to trigger 75% of the switches in Power Core.
Snow Plow	Destroy 25 snowballs in Snowblind without dying.
Snowhere to Run	Defeat the first boss.
Speed Master	Beat Speedrun Mode in under 90 minutes.
Speed P1ng	Beat P1ng in less than 10 seconds.
Spitfire	Destroy 100 fireseeds in Jungle Jump.
Stay Put	Stun the patroller in p1ng 3 times in one run.
Target Practice	Get a 100pt combo on Rex's target practice machine in the Dinonaut Base.
Tristaratops	Get 3 stars on 30 levels.
Tryceratops	Retry a level 30 times.
Turbosaur	Complete the SpeedRun mode and post a time to the leaderboard.
Tyrannosaurus Wrecks	Destroy 1000 robots.