Because dang, he was hard. Trumpeter was relentless, but Trumpeter's relentlessness was mainly from his barrage of Instant Death attacks. Red Rider...
The thing was Red Rider is mainly the fact he uses his All-Mighty skills (which happens to be the strongest of the All-Mighty skills, which even comes with the effect of lowering all of your stats)
no matter what the circumstance. Most Fiends won't blast you with All-Mighty unless your party has a certain number of certain resistances, but not Red Rider.
He uses a move that lowers your defense and usually uses his All-Mighty move around twice per turn. This means he sometimes will lower most of your stats to -3 in one round.
His Fire attacks are strong, but compared to his other moves, it is a lot easier to handle. Unfortunately, at times, he uses TrisAgion, which targets only one enemy on one of your characters that do not negate Fire. If you are lucky, he will use the party-targeting Fire skill and if you can reflect or absorb it, you can stop his turn.
So anyways, with a whole lot of restarting, I finally was able to defeat him. But my party was in shambles.
I ended up giving my protagonist a whole lot of scents to boost his stats for the battle. Thanks to his now massive Speed, there are times he can avoid my opponent's All-Mighty skills without any stat boosts.
And other members that helped in the fight:
To be honest, she isn't that strong, perhaps, and a lot of the possible roles for my stock were already taken. But I was able to give her the role of using the skill you can see at the top of the second picture. It fully revives and summons one of your demons. Clearly strong.
I actually made a new Shiva that is different from the one in my Monster-Dex.
His role is to mainly use Dekunda, which removes all negative stat-changes in your party, as a way to fight against Red Rider's stat-decreasing attacks.
Shiva also has plenty of HP as well as the passive skill that revives him to full health once in a battle, so he was able to take a lot of hits from the All-Mighty skills. Unfortunately, his MP is really low, so I wasn't able to do much other than Dekunda (used Antikuton though).
Did some damage with his "Shoot a lot"
To be honest, he wasn't prepared for this battle. But I needed any fresh hand I could use since I was losing demons each round thanks to Red Rider.
Level-wise, she is really far from able to battle Red Rider, but she is my pure Ice skill demon in my party and Red Rider is weak against Ice.
Her Ice skill did around 1000 damage and while grinning, did around 1500. However, she was only able to get a hit or two before fainting.
And a bit off-topic, but as you might know, the Red Rider can only be fought on Neutral route.
And yeah, I'm currently on the Neutral route.
Here's Masakado, a demon that can only be unlocked during the Neutral route.