Calydonian Boar is Artemis' Ultimate.
This can be done in any mode with at least 3 players on the other team but I recommend Arena.
1) Select Artemis as your god.
2) Play the match as usual and unlock 'Calydonian Boar' at level 5.
3) Get near at least 3 enemies.
4) Call in your boar and it should unlock if it attacks three enemies.