Probably most of you know this, but some may not.
I got the bonus battles 7 times already, when waiting for longer than a minute for a harrier game. 21 kills in E110 will get you an engineer starter and I got the Astron in at 5th try. Its really worth to be ready to click the bonus battle quickly, as it may put you in a harrier game a second after the bonus battle offer pops up.
I got free tier VIII premium this way, maybe you can get one too.