The most overlooked units
Have you ever had a game where you had 10+ mythics on the board all at the same time, but still somehow lost? You get so many strong mythics out there, you feel like you should win easily but you don't? This happens all the time, and it's because people don't know that a couple of the best units in the game aren't mythics at all.
These following units aren't mythic, but are nearly required in every run to win.
Using stun to increase damage
Why are these listed as some of the best units? Let's say you have a mythic that can hit an entire group of enemies really hard, but all the enemies are spread out far apart like in this picture.
When batman uses his ult or any of his strongest attacks, he's only going to hit the enemies that are right next to him. So what if we stun all the enemies so that they're all clumped together right next to batman? Take the following clip for example.
Every time batman does his heavy attack, he hits every enemy currently in the game.
Now look at this next clip where me and the other player both sell our stun units right after we had our stun perfectly set up.
Instantly they overwhelm us. None of them are taking much damage anymore. They're nearly at full health when they run past and start to surround the entire battlefield. We lost that game as soon as we sold all our stuns.
How much stun do you need?
If you have enough stun, the enemies should be almost completely locked in place right next to your attacking mythics. A very common stun layout is to have 1 full stack of the epic electro bot, and 2 full stacks of the blue shock robot, and have the same layout on both sides. Below is an example of a stun layout that is very close to that.
In this picture, we have 1 full stack of purple stun on both sides, and 2 full stacks of blue stun on both sides. Here we also have it moved more towards the back end so that our monopoly men can get more lucky coins from casting their ults while they head towards the stun we've placed next to our dps mythics. You can put stuns close to the front as well, as long as you're stunning the enemies within range of your dps mythics.
What about Coldy?
Coldy is a really interesting unit for stuns. Generally, the standard "one purple, two blue" robot stuns is the best stun setup. Coldy does have her place, but it's important to know where that place is because she has some downsides. However, to get any real value out of Coldy, you'll need her at level 6. Before level 6 she has no stun at all, and only applies slow.
The Coldy Pros:
If you do have Coldy level 6, then her role is essentially to help stun when your partner isn't helping with stun. Coldy's slow is actually impactful enough, that along with her stun, enemies will move so much slower than you can get by with fewer stun robots. The reason being, is lets say you don't have enough robots to have perfect locked-down stun. When one enemy breaks past the range of your purple stun robots, then they're only in range of the blue robots. So that one enemy is taking all of the attacks from the blue stun, and the main group is now only getting hit by the purple stuns. This snowballs, and the entire army breaks out of the stun. This ends your game basically immediately.
Coldy can fix this!
If you have a Coldy on the board, she slows enemies down enough that when the enemies are out of stun lock, they will be moving very slowly. Slowly enough that by the time they get re-stunned, they're probably still in the main group with the rest of the enemies, and still in range of ALL of the stuns. The enemies that will be farthest in the front of the pack are likely the ones that have been there the longest, and are about to die anyways, meaning they'll never leave your purple stun's range.
The Coldy Cons:
On the other hand, while she can save you on the main waves, she can kill you on bosses. Lets assume all of you and your partners biggest damage dealers are in the middle of the board, where they can hit enemies at the same time. Coldy's stun range is so massive, that she can cause one of the bosses to be stunned in your own spawn. This means you'll end up attacking only one boss at a time. By the time the second boss reaches the main area, you'll have less time to kill it, and you might not have enough time, where without Coldy you would have had plenty.
It's worth noting that you can play around this if you move Coldy during the start of the boss fight, so it's a downside you can eliminate with good play. But if you have a good partner, you likely don't need Coldy at all. So just pay attention to what you need. If you don't need her, you don't need her. But if you use her at the right time she could help you save the stuns.
Conclusion
Using these stun placements, the information in the defense reduction guide, at least level 1 sb/mg, some practice, and in extreme cases a well placed Coldy level 6, you can start to carry normal mode. With level 3 sb/mg, you can even start moving to hard mode.
Good luck!