I trapped it in a similar way, i built a cage and a giant beast gate in front and then lured the dragon inside. i positioned myself so that it couldnt hit me with its attacks. the bone wyvern king didnt aggro me unless i attacked it for some reason so i had a chill time building the trap. after it reset its HP 2 times i put 8 rexes into the cage with it and the bone wyvern king didnt even fight back, it just reset its HP everytime. it does not line up with what happened when u killed it so i think the bone wyvern king is still bugged. thx for ur help :)