Out of the 3 executioners, Kubila is the one that does most magic damage and damages relatively high. He has multiple attacks that are listed below:
Attack: He moves over to a party member and strikes twice.
Summon: He uses his scythe and cuts three lines in front of him before he moves away. The frame falls down and he summons a monster resembling a lizard that shoots bubbles at all party members.
Random Spell: He pulls out his book and casts a random spell that hits either one alley or the whole party.
No Combat: He only uses this move when he or one of the other executioners is about to get killed. He calls upon a rock from the skies on to one party member that instantly kills him/her.
Due to his "No Combat" move, it is recommended that he would be killed 1st or 2nd due to Selebus' abilities and also have some Angel's Prayer in hand for that occasion. It's also not recommended to use a spell that hits all of them at once, because he might kill all of the party members.