Alistar is pretty unfamiliar with jg and I think this actually benefits him, most enemies either don't know how to handle him or understimate him. personally I think alistar is pretty fun in jg and when played right feels very rewarding, ALTHOUGH LET ME MAKE THIS CLEAR YOU HAVE TO TRY HARD IN RANKS SO YOU MUST HAVE A GOOD UNDERSTANDING OF THE CHAMPION

The items mentioned are pretty standard for him and if you have played him even as sup they won't come as a surprise. As about for the mythics it depends of your team. If you want survivability because you are the only tank you go the sunfire, If your teams has the damage, but not the opportunities to deal it go turbo. I find myself rarely going for the forstfire, but it can be very useful when utilised correctly

The runes can vary a lot from minor changes to each teammate or enemy so I just put one that I use the most. Inspirational as the second choice isn't bad given it also hives you more cooldown reduction and the option to be in debt, like in real life, best choice of rune ever

Your only really good one is yas and I guess sylas if he steal yas ult. All the rest can be basically broken down to anyone who can deal damage and utilise you airborne and stun

Every champion that negate your combo or like garen that can hit you with his q for some reason are basically nightmare, you don't have the damage to kill them and you whatever armor or magic resist have will eventually die, so you must have someone there that can help. Now any other champion that you can play with because of your combo or q and w alone are very easy to deal with, usually champs with one or zero ways of dodging/jumping/running etc and few ways of stunning you etc are the easiest to deal with you can even 1v1 them if you leave them behind and have items