The creature mastery bonuses for defeating the Rodian creature masters give 1/100th of the damage bonus the mastery screens says they do. For example, damaging a creature with mastery level 15 (+15% damage, per the codex) gives only 0.15% more damage.
It is easiest to see this in the difference between damage dealt by creatures with high mastery and low master (like avatars, which don’t have mastery). With no other effects influencing damage dealt (no artifacts, relics, perks, or traits and all cards turned off, guild spell damage only 3.6%), creatures with high mastery only deal fractions of a percent more damage.
Testing the damage reductions is trickier, but it also seems that creatures with higher mastery only take fractions of a percent less damage than those with lower mastery.