The problem is, you have to be really quick because the daze only lasts a couple of seconds, so if you aren't close enough you won't be able to jump on them. You have to shoot them in the head with a charged shot and you should see stars over their head when they get dazed, and then you have to press R1 to jump on their back. It's very hit and miss, I've been struggling to get that trophy as well and I've not had a lot of success with mounting them (and when I do they don't do enough damage to kill the Captain).
