Ok, I see your problem. I'll write down all of my tips so u can try. This campaign took Spite a huge.... huge effort so I wouldn't stand that his topic would last for 4 or 5 pages.
HEROES CONTROLLING TIPS:
1. Grouping: Grouping is the first thing you must practice in order to "CALL" your heroes faster (because u have 12 heroes so some of their ICONS won't be shown on your left side)
There are many ways you can try to group your heroes, but the point is don't split into many groups where u can't manage them. I personally split them in
2: the #1 Group - 6 "Player 1" heroes & the #2 Group - 6 "Player 8" heroes (u can know which Player side they belong to by holding your mouse over a hero or just press Alt + A - the "Player 1" will be shown in Blue)
Team 1: Vitae, Alice, Mountain King, Priest the Healer, Fidaelis & Warden
Team 2: Ice mage, Fire mage, Necromancer, Cammy (Holy knight), Bolster & Greenstar (Keeper of the grove)
This may not be the most logical way to group but it works for me, most of the heroes in Team #1 will be the ones u need to control first when engaged in a battle while Team #2 will be used later.
2. Quick-call hero: This is the way how u can target the hero you want in the fastest way
Team 1: The Team 1 icons will be shown on your left side, so there are many ways you can choose:
-F1, F2, F3: They are the fastest "Hotkey" to target your hero, F1 = Vitae, F2 = Alice, F3 = Mountain King. So you see how quick you can save your team with Vitae? Press
"F1" to call Vitae and then press
"T" (Holy light hotkey) & use it on your team. Want to stun a boss? Then
"F3" & T (Mountain king's storm bolt), However, I avoid to use F4, F5 & F6 because my fingers may lose the accuracy so it's best for you to use F1 ,F2, F3 only, also because F6 = quick save.
-So how about the other heroes in team 1 (F4, F5 & F6?). Well you can just simply click on their icons on the left side with your mouse
Team 2: The team 2 icons won't be shown so the key to control them is: 1st Call the team by pressing
"2", then personally pick them one by one. Here, you will use the
"TAB" key (which allow you to switch on each hero in a selected team) or
"Mouse click" for faster calling.
For exam: If I want to call the "Fire mage", I can press "2" and "TAB x1". If I want to call the Necromancer, press "2" & "TAB x2". If I want to call Greenstar, I press "2" & then click his icon shown below.
Another way to call your hero is to click on them "on the screen" individually, this is a bit more difficult but u can always use the tricks above.They are very good especially if your mind is being "messed up" by too many actions to take. Just practice a few times & u'll be good in no time
3. Engagement order: The key for you to buy more time to set up your team formation, it always happens like this: Let your tanker make contact with the enemy, use "Taunt" on him and then let he take a few hits while you can have your time to cast the skill you need. If you gear up your tanker well enough, he surely will be able to take the boss on alone for ~ 10s, even when it's a powerful boss, Vitae's & Greenstar's Powerful Healing ability will take care of him in no time - Greenstar's is the best skill in this situation cuz you can still have other heroes cast their skills while your tanker is under the effect of Rejuvenation, if you tank's health is dropped down suddenly by some kind of a critical hit than use Vitae's instant healing.
=> So you see, it's not about controlling 6 or 12 heroes at a same time, but to control each of them but in a right order ^_^
Example: When fighting a boss (with Piercing Attack & some AoE skills): 1st I let Fidaelis to engage him & use "Taunt" to make sure the boss won't hit any of my other heroes, then 2rd use Mountain King's Storm bolt (5s for time stalling yoho

). And then I start to move my other heroes in an area where Cammy's AoE healing can reach all of them, after that u can start casting skills like Healing ward & Mana pool, Warden's Armor reducing... The lats things to do are to keep healing your tank/your team & enjoy the battle
4. Hotkey & Skill Usage:
There's no better way for fast management than remembering the "Hotkey" for the skills you use. Fortunately they are conveniently placed on the left side of your keyboard by Spite's intention. Besides, they mostly involve some specific keys
"Taunt" - the skill to attract the boss (Fidaelis & MK) - Always
"T"
"Healing" - most of the Healing involve
"T" &
"E" (except for Vitae's Ultimate

) like: Vitate's Holy light & Cammy's Healing -
"T", Greenstar & Bolster Healing -
"E". The Priest's Healing should be turned on auto-cast during a boss fight most of the time.
"Minions Summoning" - only 1 key
"W" - except for Necromancer's Infernals -
"C". So with a chain action of: Press
2, then
"W", then
TAB, then
"W", then
TAB, then
"C" I can quickly summon all the minions in less than 1,5s. See (Btw, I'm talking about my Grouping Formation - mentioned at 1) ^.^ ?
"Other damage effect" -
"R": You have Alice's Ultimate, Warden's Reducing Armor, Necromancer's Poison, all in one, just pick them and target "R" to your enemy
Other are some other skills like Mana pool, Healing ward, Necromancer's Shadow strike, Warden's Ultimate, Bolster's Spirit link... Still most of the keys are on the left side, u just have to remember them
Skills Usage - Saving your skill for the right time, like I said, some skills like Spirit link, MK's Avatar, Fidaelis's Howling, Priest's Armor buff have the same effect to buy more time taking damage from the boss, so use them one by one instead one at a time, or like the Healing abilities - try to heal your tank with a mix of different healing skill like Vitae's, Greenstar, Priest & Bolster so you can maintain their mana and avoid unnecessary "skill cooldown"; also remember to use mana regen item on your heroes (clarity or mana potion) during the fight
Those are the basic tips for control your 12 heroes. There are still a lot more tips for each boss/ situation, you can ask if you are having any difficulty with one. But really, it feels great after each battle - you see the boss is down at the time all of your heroes run out of mana, some almost died (or died for real lol) - that feeling definitely worth your efforts