I read most of the posts here but not all of them because I only have a short time to post right now so I apologize if what I'm saying had already been suggested.
I agree with what some people have said regarding:
I think a shop and nurse per floor would alleviate the difficulty to move from floor to floor. This would be very beneficial but wouldn't make things "too easy". But, I think lowering the monsters damage might make arcade mode less enjoyable. Though, it wouldn't be a big deal if the damage output was lowered slightly and I didn't notice much.
Moreover, I think, and I may write a separate post about this, that having more skills that support different styles of gameplay will surely alleviate some of the difficulties associated with arcade mode.
I know that tanking isn't much of a roguelike trait and I understand how integral/important blocking is in this game but I think that having more skills to support a strong, shot accepting, swordsman wouldn't hurt. Some possible skills: small heal per hit skill, higher levels for defense on hit skill, knockback resistance skill, stun resist, more health for health orb skill, more health when ressed, and so forth. Sure, not many hits can kill you, but it would be nice to have a teammate that CAN take a few hits to save the day.
Similarly, I think it would be nice to see some archery skills. Some skills might be: Ssecial attacks, more damage skills, 1 arrow pickups = 2 arrows skill, burn or chill damage on hit skill, slow enemies, magic arrow, carry more arrows, and so forth.
Lastly, more support skills are necessary for a strong team. Since this can mean so many things I'll leave it at that.
Normally I would post a lot of these suggestions elsewhere but I think that having more rounded skills and passives available to players will be a game changer for arcade mode and I think some of the suggestions I made regarding new skills could be applied just for arcade mode.
One thing that I forgot to add:
I'm not a huge fan of the continues idea but it could work if it's REALLY costly for the player. I think a good way to implement continues would be to charge a specific amount of essence depending on what floor the player has made it to in order to continue. This way, the player must weigh their options to decide of continuing is worth losing some of their essence or if they would rather restart. It kind of rewards the veterans for collecting so much essence and it keeps the new players striving to get more essence before they can start using continues. Personally, I only have 29 essence right now and if I had to spend, say, 5 essence to continue on the 5th floor I would choose to restart, rather than continue, so that I could keep saving for my next perk. Whereas, someone that has some extra essence in their wallet would be rewarded for this by being able to use their extra essence to continue where they failed.