Dhukk is not needed anymore in arena - certainly a very useful champion in general. But in arena, you already have Deacon to bring the decrease defense and since he also has the turn meter manipulation, it makes more sense to use him than Dhukk. Yes, Dhukk offers decrease attack on the same ability, but if you're doing things right, you won't really need that since you'll do enough damage to cripple the enemy before their nuker even takes a turn. And in any case you still have Duchess, who should be built very tanky as I see you're already doing, to revive the team as needed.
If you have Duchess, Deacon, Lix/Kael (Lix would be better, but Kael will do for now) and Angar/Umbral (Umbral is more reliable and 2 turn provoke, but only once; Angar could hit harder and keep provoking in a longer fight), it means you have TM boost, debuffing, damage, survivability and CC through the provokes, all in one team - that's pretty excellent for arena.
Dhukk will be more important in dungeons, where the combination of decrease defense and attack on the same ability is more needed than in arena.