The shade's attack hits everyone in my party, which is fine for my tank but not so fine for my two glass cannons- do you guys have any tips?

I usually have to use potions/mana self-heal on my cannons while fighting vierna. The alternative would be to change up the way the points are distributed to put some points in agility. =/

But not all of vierna's attacks hits everyone, so usually I can handle it with potions.

Are you aware of the way the shade fights though? She'll use your mana to heal herself. That's the main thing you'll need to focus on preventing in order to win.

What kind of potions would you recommend? Maybe I'm just not using strong enough ones?

I have a hoard of potions, so I don't really think which kinds I have xD

I use strong restore elixir III (restores 2400 health) when needed, or the elixirs that restore less health if the pet doesn't need that much healing.

Also, what I do is to go fight in lower regions until I get full health and mana, then go fight the shade. It helps some.

ah, alright. where do you get those potions again?

I get them mostly from doing missions. I think you can find some when exploring? not sure about that one.

A big thing that really helps is to go to Aryths Spire once you reach Vierna but before you start the battle. Run away from battles and collect some of those buffs. You will most likely run into explosions but if you go in to the battle with increased damage and mitigation or some other awesome combination of buffs, it is a HUGE help.
Also, use restore elixirs right after you see the shade use her mana transfusion then again on another pet that may need it after the traditional one turn elixir cool-down. This way, your finishing blows before she can steal your mana are even stronger.

Hope this makes sense and helps! ^_^

Here are some things that I usually do before I fight her.
1. Fight against mobs in the Aryth Spire until all of my pets' HP (at least 95%) and mana are full (If your pets aren't strong enough, you may have difficulty to defeat mobs in Aryth Spire so it might be better to fight mobs in lower regions).
2. After my pets' HP and mana are full, I will keep retreating from the enemies until I find a magical bottle which will give me a buff. Note that the magical bottle doesn't always give me a buff. If I'm unlucky, it will explode and reduce 40% of all my pets' HP. When that happens, I'll fight the mobs again until my pets' HP and mana are full again and repeating the process of finding a magical bottle that gives me a buff.
3. After I get the buff, I will go to Dungeon of Forsaken to fight Vierna.

Here are some tips to beat the Vierna.
1. Use finishing blow [hotkey F] immediately in your 1st turn.
2. Make sure you don't attack Vierna's minions because killing them will just increase Vierna's power.
3. After you beat her, she will change to her final form, Shade of Vierna. Now is when you really need to count your turn. Don't see the turns on the battle screen! It can be misleading.
In 1st-4th turn, just attack her with hotkey 1 and use finishing blow in 5th turn. Pay attention to the attack log (below the battle screen). If you see "Mana Transfusion" on the attack log, that is your 1st turn mark. So you'll only have three more turns to attack her with hotkey 1 before you use finishing blow in 5th turn again.
If you don't see "Mana Transfusion" on the attack log, she will most likely uses it on the next turn. Just remember that "Mana Transfusion" is your 1st mark and you'll be okay. :)
4. It's best to heal your pets after she uses Mana transfusion but you can also heal your pets if you haven't reached 6th turn.

If you still can't beat her, there are probably something wrong with your set up. If that is the case, you can read team set up guide by Divinis here.

I also put my team set up there, but my set up are spesifically for fighting multiple enemies so Divinis's team set up might be more useful than mine if you're planning to fight single enemy.