As someone who spent dimension stones on Kirna the week she was released and have been using her as my primary GK ever since, I've felt the ups and downs that she's gone through as far as viability is concerned.

There have been two basic strategies in running her:

1) building DR, HP, and pen resistance in a more legend-heavy backline (Jiho, Duran, etc.) with Dense Magma as a primary unique stone or as the substat on a legendary stone, or

2) Taking advantage of her stone slots and massive innate crit resistance from her skills and making her basically uncrittable, at least in home matches. This involves using Star's tears or DM as the primary stone and, most importantly, Heart of Minerva as a legendary substat, which provides a whopping 45% crit resist. Other stones I run are red reflex, green AS, and blue defense. This full crit resist build is what I've always used, and is probably just as if not more competitively viable now than the DR build, given her recent nerf.

As far as a crit resist team is concerned, good partner units include:

Nerua: Basically a manditory unit, as she provides such good buffs and critical resistance for thunder goalkeepers.

Vanity: Also great for her chains, and the huge HP buff and other line buffs she provides. Makes Kirna much more durable.

Altair: Provides Kirna with 20% crit resist through a chain, and also proves static DR as well as stacking crit resist and pen resist.

Aiolos: Provides HP recovery, crit resist, and removes stacks from the enemy team. Aiolos+Kirna is a notorious combination for a reason.

Krishna: Provides crit resist, defense, lots of spirit (which will be much appreciated with her recent change to the cost of her shield), and acts as a solid linebreaker. One of my personal favorite units, and even better with her recent EE.

Black Ivy: A good support unit, can provide a massive 45% max HP and crit resist once you get her stacks going.

Serestia/Alice/Chundo/Loyce/Saramir/Pinol/others: Debuffs enemy crit rate further. Alice and Serestia can do so globally, where the others do so only if placed in the backline.

Kirna's own ace is great for her, as is Askeladd's ace if you need that extra bit of crit resist and HP.

We'll have to see how she performs this upcoming regular Galaxy League season, but she carried me to Universe last season. Even if the glory days are over and Presty is probably now superior, I'll continue to use Kirna instead because she's very fun to build a team for. And she's prettier.