None of the bosses are fun imo, Titamon in particular is an exercise in "Am i gonna get stun locked by it's minions this time?" and it's rare drop is ridiculously rare for the unfun grind you gotta tolerate with him.
To add to this, with challenges such as bosses there should be a line between something that's hard because it requires you to learn and plan how to beat challenges or use different methods to see what works and something that's hard because it's based on luck.
Also item drops really should be changed to be based on the damage dealt or crits and not a chanced based on rng with any amount of damage, it adds needless grind while making it feel boring to raise a Digimon, which you can see would be a problem.
In fact I'd say the game in general has many balancing issues which expect you to dedicate hours just to even evolve your first Digimon