Hi. Set "Number of hits allowed before punishment" to 0 since you want to punish first hit. Slap plugin is not needed, it's optional.
To better understand what's going on, set "Logging enabled" to true and check /oxide/logs/PunishAttacker/
P.S. "Initial percentage of damage as punishment" is the amount calculated based on damage dealt to victim, not attackers health. But if you set "Number of hits before punishment is death" to 2 attacker will die on second hit anyways