I'm pretty impressed that you could win against Amazoness using GB. Unless you packed a lot of AMA and Cyclone, your monsters should be banished after each attack, thus no chance of activating your GB's effect.
I had luck, but not how you assume. First I needed to start first, then I needed to have defensive cards in hand. I got Nero out and Banish wasn't possible anymore for those.
But yeah there are a lot of ways to deal with Onslaught. Compared to SB now, they are easy.
Oh, that's right... Nerokius prevents the activation of effect even when attacked... That's good to know. Still, it can't be too easy getting out 3 gladiator beasts out (to summon Nero) without being overwhelmed first, so good job, if you can achieve that consistently.
As I said: It was luck. Another deck can achieve victory through damage.
I don't say it's reliable, but compared to SSA(which can make itself immune to anything except banish) or now SB, Onslaught is good handable. Just not straight attack w/o using brain.