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Neither Onslaught nor Enchantress or Bullseye have few specific counters.

If there are so many counters to these three champs than why do people keep putting them in on defense. And I don't mean most of the time. I mean 100% of the time.
Go into any battlegrounds fight and don't ban any of these 3 champs and I'll bet you anything you want that you'll see them on defense if the computer allows them to be picked.
I guess every person playing this game, and let's be honest the auto bots playing the game as well, didn't get your memo that these 3 champs are so easy to beat. Just learn to fight them. Learn to fight a R4 max sig 7 star Onslaught with your R2 War Machine. No problem!!!

If you're putting war machine in your bg deck then you shouldn't be playing bgs as you clearly have no idea what you're doing.

Onslaught is simple to fight, bullseye has many counters, the only annoying champ is enchantress and that's because of the pure rng of the fight that can screw you from the get go.

People put them on defence because you can screw up against them. Same way people put fury on defence. He's not a difficult defender to beat but 1 mistake can cost you the match.

You just need to get better at the game and stop putting war machine in your deck
mate

Also, if you haven't done act 9 yet then you're way behind on progression in terms of competing in battlegrounds. Concentrate on that for a bit