How can I get a Critical Hit?

There's an accomplishment (tied to a Plaza Ticket) for getting a Critical Hit. I'm mid-way through my third play-through of Warrior's Way, and I've never seen one of these. What do I have to do to get a Critical Hit?

Apparently from this site there are some clues at how to get them.

Having been through 50 something battles on my play through, I've only had a single one and I believe "critical strike" can only occur when you're at a disadvantage, as these are the only times I've heard of other people getting them. Basically the critical strike is a thing that can kick in when you're on the losing side to still allow you to win, even when you have fewer soldiers. When mine happened, I was at a disadvantage and had been brought down to only a few hundred less than my opponent. I don't know what the limits are for it, but I assume you have to be on the losing side for it to kick in. As for whether or not it can happen to you, my experience would say no. -Myft

I have had it happen twice. Both times had all the following true:

• Have one win and one loss.
• Be on the third battle.
• Be just a few hundred Miis below the opponent after the evaluation.

It then glowed and Critical Striked for me. - LordGeovanni

Later the person who asked this very same question gets the critical strike. He says:

Well, I managed to get it. I was facing an army more than 1.5 times my size, lost the 1st round, won the 2nd, and then I had just barely more soldiers for the third round but was at a paper covers rock disadvantage, and then the critical strike kicked in to save my bacon.

So you have to be at a disadvantage in order for the critical strike to work.

It's usually random, but you can get it to work if you are losing. When you have a tiny difference in troops (with you being at the disadvantage), it will land a critical hit.