GOLD STAR ACHIEVEMENTS (2 for 25
The following 2 achievements are obtained by collecting 10 stars on all of the levels in two entire level packs.
Respected Stuntman - 10
Collect all stars from one world.
Superstar - 15
Collect all stars from two worlds.
In each level you can earn Gold, Silver, and Bronze ranks by collecting 10, 8, and 6 golden stars, respectively. To earn these two achievements, you need to earn Gold ranks on each level of two complete level packs. Insane Hills and Stadium Stunts are the easiest so it is suggested you attempt to get all of the stars on these two level packs.
Stars are collected when your snowboarder gets close enough to them (or runs into them). To earn a Gold rank, you need to collect all 10 on a SINGLE playthrough of the level. There is a trick that makes this easier. As soon as you see a star you missed, immediately fall off your board or hit the restart button in the top left corner of the screen (on non-touch screens, the escape key can be used to pause, then hit the restart button). You can do this as many times as you want. This will prevent you from truly having to collect the stars in one go of the level, because you can play each section between checkpoints indefinitely.
Often you will need to explore a little bit to find all 10 of the stars. In some rare cases, there are actually more than 10 stars on a level, so you can miss some.
This achievement is known to be glitchy. Sometimes it will unlock before actually having Gold ranks on all levels required. Other times it will not unlock when you have gotten all the stars in two level packs. If your case is the latter, load-up Time Trial Mode and complete a few levels in Insane Hills, collecting as many stars as you can. This should pop that achievement for everyone, but it is not guaranteed.
MISCELLANEOUS ACHIEVEMENTS (10 for 85
The following 10 achievements are obtained by completing miscellaneous tasks or requirements and can be completed on any level (where possible).
AfterLife - 5
Complete level after falling from the board.
TakenGhost - 5
Grab one token after death and then complete the level.
These two achievements will most definitely come without trying sometime throughout the game, but they can both easily be completed simultaneously on the very first level of Insane Hills. Towards the end of the level you will go around a loop and then over a small jump, collecting three gold stars after the jump. After collecting the last of these stars, instead of landing normally, land so that your guy falls forward off the board. This may take several attempts, but if done correctly, he will go tumbling towards the right, collecting the star and then across the finish line, unlocking both of these achievements.
ShowOff - 5
Perform five different stunts in a single combo.
You need to perform five different stunts in a single combo. Possible stunts include the backflip and front flip, nose grab, tail grab, indy grab, perfect/good landing and rail grind. Any five of these need to be performed in a single combo.
See "Niinja Performer" for a great location to attempt this.
Crowd Pleaser - 10
Collect 5,000 points with a single combo.
See "Niinja Performer."
Great Performer - 15
Collect 7,000 points with a single combo.
See "Niinja Performer."
Niinja Performer - 20
Collect 10,000 points with a single combo.
You earn points by performing tricks. The score from consecutive tricks accumulates, as long as the time between them is short enough. Your total combo score consists of a base score and a multiplier, with your total score from the combo being your base score times the multiplier. All tricks contribute to the base score, but only back and front flips contribute to the multiplier, raising it by one for each complete flip. You need to earn a combo score of more than 10,000 points.
The best level to attempt this on is Insane Hills Level 6. Right at the start of the level there are a series of five or six jumps in which you can rack up enough score and spins on to reach 10,000 with no problem. Remember to jump before going off the jumps to give yourself more air time. Constantly being doing spins and grabs. Make sure to complete at least five different tricks to unlock the "Showoff" achievement. The achievements themselves do not unlock until you successfully end your combo by landing and stop doing the tricks.
Mr Potato - 5
This achievement requires that you dismember all limbs (both arms and legs, entirely) from your snowboarder. This is done by crashing him into saw blades which are scattered around many of the levels. He will need to slam into the blade (or blades) several times to dismember all of the limbs to pop this achievement. This will most certainly come later in the game without trying as there are many blade obstacles you must overcome.
A great location to earn this achievement comes on Level 13 of Insane Hills, called I Saw You. About half way through the level, you will come to a massive saw blade that you must go around. Just let yourself fall into the blade and you will have no trouble losing all your limbs. See the video below for the technique.