Since I’m using Nina as a second character, I’m using these gems:
Power gem - Your partner performs a launcher
Power gem - Connect with 4 special moves
Meter gem - Your partner performs a launcher
The idea is to have Poison tag Nina in by using her universal launcher, since it has a lot of range, and maximizing the damage out of it. The following combo - c.HP xx Geyser Cannon, EX Skull Splitter, Rolling Dash grab - activates the second power gem and does around 400 damage, from then on I have all of her gems activated at the same time and a massive power and meter gain boost.
My thought is that as long as you have meter, you can do a block string cancelled into her ex unblockable command grab. Damage isn’t the best, but it nearly always hits as it’s hard to react to it and jump or reversal. I think cross gauge gems are valuable for Nina for this reason.