In fact it is very important to grind the arenas as a gold income later on in the game. There is nothing stopping you from repeating the arenas every month that you have already beat and the ring of death is by its nature a never ending grind for exp and a small amount of loot. Otherwise you’ll have to do a few playthroughs, learn the missions, prepare for them, come back to it again next “new game” you do and try again. Overall to answer your question “yes you can” but only in a certain time limit. It’s a great game just needs a bit of balancing in my opinion. And my reroll cost is so high already its stupid. For me it was a long time and basicaly impossible to get a necromancer without 'no exp' gain. Combine that with downtime on missions and nothing to do you can get coin stagnant at times. The gold cost for rerolling items/gladiators is stacking infinitly and makes it very hard for example to get a decent roll for gladiator classes if you consider the number of diffrent gladiators and the number of diffrent quirks they can have. There are limited items and you have to pay to reset the market to get more items. That being said the item market is “randomized”, so you can’t just select what you want. Stack 6 of those on a shield warrior that blocks and they get good health to tank the boss. You can put 6 shields that give 720 health (heavy shield think it’s called) once you reach a certain level to unlock it.
So you can have say max of 6 items on a gladiator once you upgrade your building to max level. So you’ll have to scramble to try to train them, do arena fights or something to get their stats up or play the market selling resources to get gold to then in turn purchase better items to stack them on your gladiators. That being said missions are on a timer and the bigger bosses that are super hard have a longer timer but they still do time out.
There are many times where enemies are “super powerful” and you might have to play through a few times to understand what builds are good, how to get stats for certain fights and to make sure you have a good balanced team. This shows how to grind when the enemies are to strong and come back later, or can i only fight an enemie once.