Well, here's a suggestion, maybe you'll like it:

Resilience of the Way (Ex): Starting at 9th level, a kensei receives energy resistance for each Way path he has unlocked. If he has access to abilities from the Way of the Earth (you would update these to be the full names, of course), he gets resistance to acid, if he has access to the Way of the Wind, he gets resist electricity, if he has access to the Way of the Waves, he gets resist cold, if he has access the way of the Flame, he gets resist fire, if he has access to the way of the void, he gets resist force.

The kensei's resilience comes from his natural physical condition, as well as his stamina's connection to the Way. The amount of energy resistance he receives is equal to 5*his Constitution modifier (minimum 5). If this would give him 55 or higher energy resistance, he instead gains energy immunity.