this is a quote from tibia official manual
"To help a guild or party member in a critical situation, you can join the same fight. To do so, right-click on the other character and select from the context menu "Join Aggression". This way you are instantly transformed into the same PvP situations like the character you have joined. This means all these characters will receive damage from your area spell or be blocked from your magic wall, without the need to be aggressive with all of them beforehand. Be careful though, as this counts the same as if you had attacked every single of these characters yourself."
"If a character is killed unjustified, its death has a certain value. Characters that have helped bringing this character down in some way will receive a certain percentage of the death's value. Characters who have actually dealt damage to the killed character will naturally receive the biggest share. However, also blocking or debuffing the victim as well as healing the attacker will fill up a character's bar of the unjustified points widget. As the system is additive, characters which have e.g. dealt damage to a character and blocked it will receive a higher share than characters who have only dealt damage."
so yes you can red skull by just using mas res if you have wrong expert mode on.
that is one of the differences between pvp and retro pvp
at retro "supporting (healing, blocking etc.) an attacker does not lead to any form of punishment"
check these here