To easily beat the Norse Hammer, pause the game and press Square. For some reason this allows you to end the boss fight.
It's not mentioned in the manual, but if you press jump and triangle at the same time, Obelix will do a body slam to the ground causing damage to everything but bosses.
Beat the game on any difficulty to select a different costume for Asterix.
Jump into the middle of the Battalion and repeatedly press X until they explode.
Repeatedly press X until he disappears.
Approach an enemy, hit them once, then press Triangle. Asterix will swing them around. Have Obelix throw an enemy towards you. If timed correctly, you can press Triangle and swing another enemy around, hitting the other opponent. A 10X modifier will appear.
Press L1(2) to have Obelix swing and throw an opponent to Asterix. Hit them before they touch the ground to instantly destroy them. Hit the opponent just after Obelix throws him (while they are in the air), and Asterix will glow and a 3X modifier will appear.