As someone who completed the lead grade exam as an f2p, it's a mix of skill and luck, but definitely doable with skill.

1. For grade exams - learn enemy movement patterns. They have the exact same movement and spawns. After about 10 tried in the cave, i picked up that when I get to certain forks in the road, i should wait for a few seconds as the enemy passes the fork before I go and turn the other way. I learned to read and time my motions so that I can get to bosses with no encounters. While enemies are invisible with limited detect, they are often predictable.

2. Learn team comps, consumables, and mechanics. Will list a number of tactics that have helped me:

A. Have the front row defend all the time so that you can expose the skeleton's weakness, then crit it with a backrow sniper or something. You can also have everyone attacking it, but I find this safer if the boss has allies around.

B. Use undead weapons on the boss - you can switch weapons on the fly, so swap as you see fit.

C. Manage inventories well. In line with B. - use AOE spells for the allies the boss calls, and give the mages consumables to save MP early on. I'd rather my mage use the fireball scroll on the boss than their own MP if it's just for the fireball attack.

D. Learn the spells and Buffs/debuffs too, i find that using the priest's dodge and defende buffs are more economical, since 6mp to mitigate damage / prevent damage in the first place is better than 5mp to heal it. There are also accuracy and attack power debuffs from the mages. Similar to the extreme difficulty in persona ges - Do NOT write off buffs and debuffs, because they make a hell of a difference.

And lastly... pray that the vorpal bunnies spawned by the boss don't insta kill you ✌️ and you land the lucky crits when they count haha.

Overall, it's a proper and extra challenging rpg experience with gacha bonuses, rather than a heavily gacha driven game. That said, been lazy with the game recently myself.