Ducking or performing a dodge roll to evade an attack the moment before it hits will activate 'Gleam'.
You will know it has activated as a red symbol will appear and your character will have a glowing outline a a few seconds. Rinse and repeat 50 times for the achievement.
This will probably come naturally as you play but if you want a good place to grind it out I would suggest any of these three methods.
1) Dodge the fan projectiles from the regular enemies that throw them (any location)
2) In the secret location in stage 7 where you get the secret achievement "A Foot Face?" keep ducking the projectiles at the platforming section.
3) Against the final boss dodge under the skull projectiles he sends out.
Note: Whilst Gleam is activated you deal extra damage