Battleborn

TL;DR: All Missions in 2-Player Coop with Thorn and Pendles (DLC) on "normal hardcore".


I've played the whole campaign on normal hardcore in 2-player coop, except for mission 4 (The Archive) because you need to play this mission on advanced hardcore to get Thorns Lore Challenge anyway.



Therefore I can confirm that you can mix all modes. I did 7 missions on "normal hardcore" and because of the above mentioned challenge I did mission 4 on advanced hardcore and still got the trophy as soon as I completed the last mission.

So, for the hardcore missions, we picked Thorn and the DLC Character Pendles (purchasable with ingame-credits, if you don't want to spend real money) in every mission and they were all surprisingly easy.

Thorn, in case you don't know, is an archer and you can aim down the sights, which gives her an impressive range, which is one of two keys in our strategy. As soon as you have an open area you can snipe most enemies and sometimes they wont even notice that their fellow soldiers beneath them are dying one by one. In smaller areas using cover helps a lot. (You may pick Marquis, if you like him more. He has the same long sniper range, he should work too. Thorn has some sort of bullet drop, Marquis as far as I know not. But I think, Thorn can handle close quarters combat better, if she is forced to do that, that equalizes her bullet drop). If Pendles is into the action, he will draw most of the attention and Thorn can snipe heavy enemys (especially the guys with the big steel-shields) from behind.

Pendles is a melee assassin with a passive permanent stealth. As long as you don't attack, you will be in stealth with no time limit. His stealth is activated automatically as soon as you break the line of sight to enemies (walk around corners, walls, great objects and so on) or if you hit your smokebomb-skill. This passive ability is the second and most important key in our strategy: If you stealth, Thorn will get all the aggro and if she dies, the fights ends. No one will come and look for you. There is no detection-range, if you come to close to an enemy, you may even take damage in stealth. You can even bump into them, they won't react! This stealth ability is incredible powerful!
So, if Thorn dies, simply cloak, go to her and revive her. Don't deactivate your cloak until your cooldown for the smokebomb is ready again, even if Thorn dies immediately again. Revive her once more. Never fight without the smokebomb-ability ready. Play safe, thats the trick.

Pendles is a mid- to lategame-character. He starts relative weak, with no health regeneration early on. If you are low on health, stay in stealth, let Thorn do all the work. Remember: If you die with Pendles, Thorn has to run all the way to you, trough all enemies, under constant fire. That's much more riskier, then reviving her!)
As soon as you hit Level 3, you can get the 15% Lifesteal-Helix for your Basic-Attacks and with Level 4 an even more powerful Lifesteal for his Injection-Skill. With Level 5 you then get your ultimate, which is capable to do damage while you are stealthed, or if you like you may attack with your other skills or basic attacks while the ulti is active. In this case, the cooldown of your Injection-Skill is reduced to about 3 seconds, which means, you may use Injection (which should already be upgraded to that nice lifesteal!) 3 or 4 times during the duration of your ultimate. He also has some helixes to increase the damage done if you hit the enemies in the back, while Thorn is "tanking" them from very far away. If you have played Pendles for a few rounds, he will also get a helix-mutation, which reflects 15% damage back to melee-enemies, in case you are tanking and Thorn is sniping them in the back.


Equipment:
I cant recomment specific items because there are so many combinations or orange items with cool effects and everyone of us has got other items.

But in general:
Thorn always took some life-regeneration items and a shield-generator. (Thats right! If you didn't already know: Thorn and all other Eldrid-race-characters in Battleborn don't have shields, but passive life regeneration instead. However, if you equip them with items that increase their max shield strength, they get a shield with that amount of HP! This shields are weaker then regular shields, of course, but compared to the slow passive health regeneration, look at them as "fast regenerating hitpoints"! A weak shield is much better than no shield!

On Pendles I always took health-regenration-items and damage-reduction-items except for the Defence-Missions 3, 6 and 7. Then I used items, that passively generate shards (they have an cristal as icon) and a wrench, which reduced the costs of turrets and drones. As third item I took an orange armor, that gives +150 HP to all team members, for better sustain for Thorn and myself.

Thats it :)

And there is one last thing, that is hard to describe and I have no evidence for it: Oftentimes
it seemed, that the enemy AI and the scripted triggers for certain events were not intended for such a character combination. For Example in Mission 3 at the end, where you have to rescue Caldarius and need to defend his power core from waves of enemies. Our Thorn stood on the roof of the building, to snipe the enemies on the landing-pads and destroy the anchors that spawn directly on the rooftop. I was inside, near the powercore, in stealth. Usually this part is very, very hectic and the enemies rush us from all sides. (You don't belive how often we lost this mission earlier on normal, without hardcore, with a lot of character combinations. I hate this mission!) But in our particular hardcore-playthrough we had no trouble at all. A few enemies rushed in, but attacked my temporal drone and the defense turrets (flash for stun and thumper for aoe damage, btw), but they never had the intention to damage the powercore. We finished this mission with 100% HP left, we were never that good ever before. Even better, most of the enemies, that were flown in by the troop-transporter ships did nothing than standing around on the landingpads and get shot by Thorn from the roof. One by one, they didn't react xD

We noticed such AI behaviour in nearly all missions in one or more instances. It seems, that some triggers require a visible melee character to be on a specific place, and Thorn was always to far away and Pendles seems not to count to register and activate the events xD
That was very helpful! ^^

As you may have noticed, English is not my native language, so please excuse any mistakes ^^"

Please don't downvote because I don't have the trophy: We did this on Xbox One.
I hope this wall of text helps you in some way. Good luck!