Boss multiplayer difficulty scaling

[Bumber64]

Some bosses tend to be difficult in solo, but become increasingly trivial with extra players. The only real solution to this issue is to scale boss durability, simply because having 6 Wolfgangs clobbering a boss shortens a fight to the point where there's not much chance for retaliation. (Dragonfly is particularly bad with her stun mechanic.)

The scaling can take the form of extra health or defense. Either are essentially the same, though there may be some slight difference in how defense calculations work.

How it would operate is that if a player is close enough (e.g., boss fight theme range) then the boss dynamically increases its difficulty level. When they leave, it goes back down. The health component is designed to maintain the old health as a percent, so there's no problem changing max health mid-fight. The increase in health or defense should be non-linear, allowing players to be present for support roles without making the fight too difficult. Probably a logarithmic increase, with each extra player being slightly less significant.

At certain thresholds, we can increase other aspects of the fight's difficulty. With 4+ players, we might want certain bosses to summon extra minions to keep everybody occupied. Maybe some ability cooldowns could be affected, but we should be careful that the boss isn't picking off a single targeted player.

The added difficulty should be justified with greater rewards. If a fight was kept above a certain difficulty for enough damage dealt to the boss (e.g., 90%), the boss should drop bonus loot. (The extra loot can be dropped out of a portal so we needn't worry about deerclop's extra eye.) We don't care if the boss healed the damage afterwards, as long as the effort was put in. We'd keep a table of how much damage was dealt at each N player scale and determine the max level it qualifies for at the end. Also non-linear; you aren't getting N deerclops eyes for N players.

We need to consider player ghosts. On one hand, we don't want a death to force players to stop attacking to maintain bonus loot due to scaling down. On the other hand, the ghost isn't contributing to the fight. What we can do is require the ghost to move away to scale down, but not scale back up when a ghost enters range. This gives players some control over the difficulty scaling, while still allowing spectators.
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One alternative to tracking the number of players is to add explosion-type resistance when too much damage is dealt in a short time frame. This is simpler, but has drawbacks. In particular, it punishes players for doing too well, and allows support at no cost (e.g., healing and killing minions). You wouldn't waste resources bringing bunnymen to a fight if their damage was restricted, and then they died halfway through. It might also cause problems with needing to cancel Crab King's healing, and is antithetical to DFly's stun mechanic. Given that, it's important to keep the concepts of DPS and player count separate.

Edited February 1 by Bumber64