You must fight 20 tonberries in-a-row. So if you wander off and fight some random Geezard in between you're unwillingly reseting the counter to zero.

My method of killing them is using Kamikaze and reviving that character the next battle. It works so much better than Renzokuken. Another good method is to refine the Chef Knives into AP ammo and use Irvine's Shot

I'm not sure what CMP means by reload but I've managed to save my game, turn the power off and keep the count intact