Bukkit

19:06:50 [SEVERE] Could not pass event EntityDamageByEntityEvent to MobHealth v7.1.0
org.bukkit.event.EventException
        at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:427)
        at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62)
        at org.bukkit.plugin.TimedRegisteredListener.callEvent(TimedRegisteredListener.java:30)
        at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:478)
        at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:463)
        at org.bukkit.craftbukkit.v1_6_R3.event.CraftEventFactory.callEvent(CraftEventFactory.java:94)
        at org.bukkit.craftbukkit.v1_6_R3.event.CraftEventFactory.callEntityDamageEvent(CraftEventFactory.java:398)
        at org.bukkit.craftbukkit.v1_6_R3.event.CraftEventFactory.handleEntityDamageEvent(CraftEventFactory.java:423)
        at net.minecraft.server.v1_6_R3.EntityLiving.damageEntity(EntityLiving.java:644)
        at net.minecraft.server.v1_6_R3.EntityMonster.damageEntity(EntityMonster.java:39)
        at net.minecraft.server.v1_6_R3.EntityHuman.attack(EntityHuman.java:889)
        at net.minecraft.server.v1_6_R3.PlayerConnection.a(PlayerConnection.java:1158)
        at net.minecraft.server.v1_6_R3.Packet7UseEntity.handle(SourceFile:36)
        at org.spigotmc.netty.NettyNetworkManager.b(NettyNetworkManager.java:230)
        at net.minecraft.server.v1_6_R3.PlayerConnection.e(PlayerConnection.java:117)
        at org.getspout.spout.SpoutPlayerConnection.e(SpoutPlayerConnection.java:195)
        at net.minecraft.server.v1_6_R3.ServerConnection.b(SourceFile:37)
        at org.spigotmc.netty.NettyServerConnection.b(NettyServerConnection.java:132)
        at net.minecraft.server.v1_6_R3.MinecraftServer.t(MinecraftServer.java:594)
        at net.minecraft.server.v1_6_R3.DedicatedServer.t(DedicatedServer.java:240)
        at net.minecraft.server.v1_6_R3.MinecraftServer.s(MinecraftServer.java:483)
        at net.minecraft.server.v1_6_R3.MinecraftServer.run(MinecraftServer.java:415)
        at net.minecraft.server.v1_6_R3.ThreadServerApplication.run(SourceFile:583)
Caused by: java.lang.NoSuchMethodError: com.garbagemule.MobArena.waves.MABoss.getHealth()D
        at me.sablednah.MobHealth.API.MobHealthAPI.getMobHealth(MobHealthAPI.java:265)
        at me.sablednah.MobHealth.ServerDamageEntityListener.onEntityDamage(ServerDamageEntityListener.java:155)
        at sun.reflect.GeneratedMethodAccessor290.invoke(Unknown Source)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:606)
        at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:425)
        ... 22 more

What version of MobArena are you using?