Gemini has 2 things going for it. First, it has a status that can allow it to do 20% more damage on targets with a status affliction. Second it has a one turn buff that lets it do 30% more damage. It's also got an attack with power greater than the EB it has, and at the very worst case, can follow one up with the other, to ensure the quick deaths of 3 party members, resisted or not. Sunset Empathy has a power of 750, requires 6 element, hits 3 random targets; compared to the EB that needs 9 element, power 640, but hits all targets. If it has both the status affliction damage boost and the 1 turn damage boost active at one time, it could spell death on an EB. This is why I consider it a bigger threat than Aries. (And also the reason why I summon it a lot more than Aries)

As for Leo not using guard, I'd say its less trigger happy about it. It used to be that as soon as it was available, like the very first turn, it would use Guard. That was great for me, because I couldn't get an EB off in Turn 1, but I certainly can in turn 2. Now I have to hope that it doesn't use its EB on Turn 2, otherwise, I've just lost half my damage right there - it's getting better at knowing when to guard your attacks.