Almost forgot to post this.
Here's Tezcats FLB.
As you might be able to tell, it's really underwhelming.
The way the buff works is as follows. You summon Tezcat and in addition to it's usual effects he gives you an undispellable buff. The next time you summon a Tezcat AND the enemy has full charge diamonds you are guaranteed to get the 100% echo. It doesn't need to be the same Tezcat. So at best you can get it at turn 9 with a friend summon. I have not tried out if cross-summoning works. That would be the only upside for this uncap.