Giving a bit of life to what is otherwise generally used as a costume instead of as actual armour, you'll be quite literally fighting knights in (slightly modified) Breaker gear, and matching weapons.
Regardless of Tier, each knight has an individual health that decides, both based on it's own health and upon it's teammates, it's own attack pattern.
Simply put, they'll tend to melee you off the bat when health is high all-round, but switch to bombing around the edge and keeping you towards the center when their health runs low while their teammates are still high health.
When their health is high, but a teammates' health is low, they'll start retreating a little, keeping their distance a bit better, and occasionally starting to gun you down instead. As their teammates' health values drop, they'll gun more and more. When they've got high HP but both teammates are down to bombing, they're gunning around 50% of the time.
And then when they're all down to lower HP, they'll start to gun you much more often. The weapon they're going to fire is visible just like a knights' own, and this plays a more important role in Tiers 2 and 3.
The knights in question carry armour a helm, armour and a shield that come from the Breaker kits - 1* for Tier 2, 2* for Tier 2, and 3* for Tier 3, but the actual stats are modified to fit the Tier, being purely for looks. Additionally, unlike a normal Breaker piece, they carry a slight aura of their own element in Tier 2, which becomes more obvious (but not obtrusively so) in Tier 3. Unlike normal Breaker gear, too, they are not weak to any element, only resistant to their own.
Their weapons match their armour. Fire-dealing damage for the fire-resistant gear, shock for shock and frost for frost. They carry weapons from which the * is equal to their armour. Graphically speaking, this'll introduce the Starter Pack's shock bomb into everyday gameplay - but that doesn't mean you can get it from them.
And lastly, their eye colours - a strong red colour for Fire; solid, fairly dark blue for Shock; and a much more pale blue for Frost.
Now, unlike normal bosses, you can see their health in the top-left corner of your screen, under your minimap. During battle, the Current Mission box is moved to be underneath your current party. Except in the last stage (or until they're all gunning in Tier 1), they're invulnerable once they reach one pip of health left. They'll glow slightly pink like Royal Jelly minus the shield to represent this. While doing this, they'll also attack less often - the slowdown is determined by Tier, ranging between 50% and 15%
In Tier 2, you have to fight them twice and in the second stage spawners will appear. These spawners are somewhat slow but increase over time until they're on-par with the spawners in Ironclaw Munitions Factory, so be quick to dish out the damage or you'll be fighting a losing battle. Bombs, as well as having an increased radius, will be placed slightly more randomly along the edge rather than in a uniform square like in Tier 1, their guns will split upon hitting the first time like an Alchemer line gun, and their swords will occasionally shoot a small, temporary AOE attack (which, unlike Greavers, does not penetrate shields and does not guarantee a status effect every hit - though it will do heavy damage, so stay away). For the effect of this, imagine an Avenger's charge attack, but imagine it disipates instead of shooting off and does so more quickly than the actual Avenger. They will, rarely, also choose to charge their gun. The weapons modifications are active throughout the fight, but when in Stage 2 you can kill them directly rather than whittling them all down to gunning first. I wouldn't advise doing this, however, as their swords are considerably harder to dodge when they're running up to you than gun projectiles are from a fair distance.
In Tier 3, A third stage is added. This isn't just slowing gameplay down, I swear. As well as their weapons generally improving between tiers, they gain another bonus in Tier 3. Chromalisk status gels. When bombing, they'll occasionally choose to run to the center, completely invulnerable, and place down a status gel of their own element - though they take no damage, you can simply hit them a couple of times to knock them down, and they'll get back up and run to the edge again. Bombs are also much more varied when placed along the edge of the level, guns will split of into 4 rather than 2 and will semirarely split off if the first split connects with an enemy. The second hit-off will never split, though. They'll also charge their guns a bit more frequently, but still fairly rarely. The AOE effect of the sword lasts slightly longer, but still less than an Avenger's initial sword projectile at this point. The bomb modifications are active throughout the fight, while the gun and sword mods take effect in Stage 2 onward. Like in Tier 2, you can knock them out while the other two are still at full health in the last stage, but doing so isn't easy.
As to the levels leading up to them, and the boss rewards? I have absolutely no idea.
"Health determines attack pattern."
Finally, something other than "attack until you drop". The main issue we have regardless is creating an AI to follow since they are not going to act like Mecha Knights. That would be stupid.
Having them use swords at first to rush until their health drops makes sense somewhat, but unless they start somewhat close this seems unlikely. Having them switch to bombs because their allies are low health is somewhat situational depending on what weapons they are using to begin with. In fact, all the weapons they use determine whether this would work.
For those who do not know what Breaker equipment refers to see here for an example of one. They have resistance to one status but weakness to two others, all have split normal-element defense. Since they are all status based, they may as well use status relevant weapons as you state; however, which weapons would they be using? They would use a sword, gun, and bomb of each of their respective statusesuses; fire has Fireburst for a sword, Firotech for a gun, and Fiery Vaporizer for a bomb; fire, freeze, and shock each have a variant of these three weapons. Using the dead end 1* bombs for T2+ is not viable because there are no such variants and should not be. For the shock enemy its color may be green to give variation amongst the colors. There are several other weapons which inflict statusesuses which may be applied to their arsenals but to keep them equal would mean retaining Brandish, Alchemer, and Vaporizer lines.
Seeing enemy health has been discussed and defeated because it neglects the intent to keep their health vague. Users are supposed to learn how to gauge health remaining from experience. Forget copying the invincibility phase, stick to what it already has; swords at first, bombs when their ally is damaged to cover them, guns when their health is low.
Spawners. This is the only time I have seen someone refer to spawn pads as that. It sounds like a fish or something. Maybe I am crazy. Stick to having them increase in difficulty based on their equipment. In T1 they can use the 2* variants of the previously stated weapon lines, T2 3*, T3 5*. Alternatively they can use the 1* at T1; for the fire one it would use Firecracker in T1 but in T2 it would use Fiery Vaporizer Mk II.
Using the Brandish, Alchemer, and Vaporizer lines would be ample complexity so long as there is no friendly fire on their part. If they have the same behavior mechanic amongst all tiers the real difficulty would boil down to having the equipment to defend against the damage and statusesuses and the weapons to counter theirs. Seeing as they deal element damage and wear equipment against element damage they would essentially be gremlins again in terms of resistance and damage type dealt. Instead of having all three simply be status variants of one another there could be three completely different ones, each with one of the specialized damage types: for pierce it would use a Flourish line, Barrage, and Callahan/Blitz, for shadow it would use a Nightblade, Graviton, and Umbra, for element it would use a Brandish, Alchemer, and Vaporizer with each being a different status. The pierce one would be primarily offense based, seeing as it would use offense based weapons. The shadow one would be more for kiting, using Graviton in junction with the others to ambush and/or chain immobility for the team fighting the trio. The element one would vary depending on which weapons used. For its weapons I propose Combuster line for its sword, Shivermist line for its bomb, and Storm for its gun.
The elephant in our room is still the AI; how they work, what they do, and where is that bucket of mud I had a few minutes ago.