1st idea
get the right Artifacts.
for example the champion who kills the boss gets the artifact in the way the champion is build , for exaple a defens type champ gets an defense based artifact, same for attack type or hp type.
2nd idea
Some Bosses have certain artifacts they fight with which can be earned at the end when winning the fight.
for example a special shield or Gloves a Banner etc. that could be imagined trough the fight (what is strong or special on the item etc. ( or you see the artifact before the fight : ) )
( this would be also god for the artifacts you offer to buy ....not so much gambling stuff where you dont know what you get , better to know for what you fighting or paying for )
3rd idea
extra arena place , where you can battle for Arifacts from the opponent.
the leader champ can choose what artifact to place before the fight.
so everybody would pay attention against whom to fight because you can loose an good artifact