To easily get the achievement of "Hexic Addict" and "Complete 100 games of 
Hexic", go to Time mode and start a game on the Normal difficulty setting. Get 
at least one cluster for the clock to start. Then, wait for the time (50 
seconds) to run out. This counts as a full game. You can do this as many times 
as needed to get the "Hexic Addict" and "Complete 100 games of Hexic" 
achievements.

To get the achievement of "Flower Child", you must form a flower cluster. The 
easiest way to do this is the wait until you see a partially formed one (four or 
five pieces of the flower already together). Then, drop in the last piece and 
you will have created a flower. Flowers are worth a lot of points. Make a flower 
whenever you get the chance to do so. Note: Do not destroy anything beneath a 
partially formed flower, or the flower will break apart.

To get the achievement of "Star Gazer", you must form a star cluster. The 
easiest way to do this is look for a partially formed star cluster (two pieces) 
and just drop the last star piece in whenever one gets close enough to the 
cluster. Also, the pieces in a star cluster do not need to be the same color; 
they just need to be star pieces. Forming a star cluster also gives you a lot of 
points and usually a large combo. Note: Do not destroy anything beneath a 
partially formed star cluster, or the star cluster will break apart.