Mental agility is mostly tested in stage two, I tried to prepare for it myself but couldn't find anything and used the links given in this forum. NATS has specifically designed the tests to be unfamiliar to people taking them so that in theory everyone is in the same boat for this stage. And I did give you a bit if an insider tip in the last post if you didn't notice??
Stage one - I prepped by reading the pre-learning material they had given us. Didn't do much else apart from that.
Stage two- looked at the links for games on this forum
Stage three - looked up competency based interviews.
I did try looking for more info on the second two stages, but there isn't much out there apart from what has already been discussed, so it would be better to focus on practising what you already know will be on there.
Preperation does not make passing a certainty, it will help but there are never any guarantees. Good luck