I went pure STR for Kymael because I wanted to maximize her shield. I even gave her a Base Atk skill card. Her ATK is about 12.5k now and she has about 90k HP in Labyrinth with the STA buff from Noble Chivalry. I do wish she had a little more HP, so I might swap out one or two of her rubies for opals at some point, but at least for now I'm pretty comfortable with the mix. Highest shield I've seen was about 35k HP when she casted the shield during a SSS Attack round buff in Labyrinth. She may or may not have had Blackaria's ATK Buff on at the same time.
Interesting thing is that using Kymael's shield one could figure out exactly how much that ATK buff is since the shield is based entirely off of ATK, so just take into account her currently active buffs and do the math.
Good point about the shield. It definitely makes a difference, but it also tends to drop at the worst moments...
I'm curious - what level of gems are you using? All SS-SSS?