Slayers feels a bit like a wasted opportunity for me. The majority of people play it just as if it's survivor mode and ignores the AI, which goes against the whole point of the mode. It's especially annoying in team slayer where you can't simply lead human players to eachother so they fight eachother and you can just run away. Another problem is the score system, bosses are simply worth WAY too much, and it's made even more ridiculous by the 2x bonus if you're on the losing time (that rule definitely should have been removed from 2vs2 matches). So if you're on the losing side and kill the red executioner on prison, that's 30,000 points. Which is quite a lot.
I did a bunch of random tests yesterday to try to figure out how the scoring and netcode works.
Regarding coop I'm very sure the hit detection works in a way that if you see blood on an majini, that means you hit him and then the client will just send "Hey, I hit majini B in the head, make him die now please!". So no matter the latency you can always aim directly at enemies and it'll always register. But it does NOT work like that when firing at human players in versus mode. I'm pretty sure it works in reverse actually. It's the person who's getting shot who will send a "Okay, I got shot, lower my health now." packet to everybody else. Which means it'll be impossible to shoot a moving target by aiming directly at him if your latency is high. On the bright side, as the guy being shot at you will ALWAYS know when and why you get damaged.
I'm also pretty sure the game is p2p but still depended on the host (host is probably responsible for managing AI, items and such). So you have to deal with an individual ping against each player instead of just the host. Since so many things are handled client-side this game seems like it would be the perfect victim for hacks or people deliberately lagging their own connection so they'll start teleporting around.
Things I learned from random tests: (when I mention scores for shooting human players, it's about survivor mode)
-This is very obvious, but when playing slayer, pay close attention to the scoring chart, killing other human players is NOT worth a lot of points. Killing 3 normal majini is worth more points and is a lot quicker.
-When you're grabbed by majini the first bullet which hits you will deal NO damage but will break the grab. If someone melee's you, they'll deal about 1/4 damage of your health bar and get 1000 points. This isn't very obvious, but it's a pretty important thing to be aware of. If you're shooting at someone who's grabbed, make sure to keep shooting directly at them to make it worthwhile, shooting at them once with the magnum would be a huge mistake.
-During certain animations you will not go into dying state when shot but the player shooting will still get points. For instance when climbing ladders.
-Frag grenades or rocket launchers don't have a big radius. Beyond a certain radius the player will get stunned and take no damage, but you will STILL get points (which is quite a lot considering the 3000 points from rocket launcher, and can be lot from grenades if you throw several at a time). Edit: Actually, you only get half of the score if the player is merely stunned. But that can be still very significant if you hit several players at once.
-Doing a melee on a player in dying state will always give you 4500 points. But shooting a guy in dying state will give you 3000 points + the score of the weapon (for instance, 3100 points when killing with a handgun). With the exception of 3 weapons you'll always get more points by doing the melee. The exceptions are magnum which would also give you 4500 points (3000 + 1500). Rocket launcher and rotten egg would give you even more (around 6000 points).
-Speaking of which, there's a few animations which will deal damage to all players even though it isn't immediately obvious. For instance, you know the animation where the axe guy grabs you (and if you fail the QTE) he'll throw you to the ground and instantly kill you? If other players stands near the axe as the axe guy does the last part of the animation, those players will die too. Similarly, player's melee animations works the same way. If a human player does their haymaker (or similar) melee it'll deal damage to all human and AI players which are in range which is very significant considering it deals a LOT of damage, and you get a nice 1000 points for every human player hit.
-One such scenario is when a player is being healed from dying state. If you heal a player while in dying state there's 2 different animations. If you have no herb or spray, you'll give the player a quick adrenaline boost and you are 100% invincible while in this animation, no enemy or human player can hurt you. On the other hand, if you heal with a herb or spray, BOTH players are vulnerable. So imagine a scenario with 1 player in dying state with his partner nearby, but an enemy human player is close too. The enemy starts doing his haymaker while 3rd player heals his dying teammate with a herb, then the haymaker will hit BOTH players and get 2000 points. And the dying teammate who was healed will probably go right back into dying state. If you're playing against very laggy players that might even happen when healing with an adrenaline boost.
-As with mercenaries, there is actually a limit on how many AI majini there can be in slayer (not sure if it applies to survivor mode too). When playing 1vs1 (the limit might be higher with more players), no more enemies will spawn after you've killed around 150 of them.
And that's all the info. There's definitely some potential to exploit the scoring system, but it actually has little effect on the actual gameplay. Most of the exploits are so situational that you'll maybe be able to do one of them once every 10 matches.