I've finally managed to reproduce this one.
1. You need a toon that matches a trooper's toon. (I used allies, eddies are untested).
2. Battle with your toon, I farmed LOL X.
3. Swap your toon on the next battle with your troopers max skill shard toon.
4. On the following battle swap your non max skill shard toon.
5. Viola, your toon now has max skill shards for calculating damage.
Happy exploiting.
This exploit took a LONG time to track down, I would suggest to the devs to make a post whereby the community is shown how damage is calculated under all conditions. I know they don't want to give us this info, but I'm guessing there are many more bugs like this that have existed since release and gone unnoticed.