I'm level 17, about to be 18. Starmie with 934 CP is my hardest pokemon, although it only has 69hp and I've got a hypno with 881 cp and 95 hp so I dunno which is actually harder.
When you go and do battle at a gym how many times do you have to fight to take it?
I went up to the gym before and twatted three or four but they just kept coming.
Each gym has a prestige level which defines how many pokemon can be in there defending it, that is in the info bar on the top of the screen when you are in a gym, it says what level you ar on and the
n under that has a number of dots and crown, this shows how many pokemon can be in it, if the spots are black they are empty, white they have a pokemon (so if your team owns the gym you can know if you can drop one of yours in there to defend it).
The main bit of the screen shows the first defender, too see the others, swipe left.
When you battle a gym owned by an opposing team, each fight you win will reduce the prestige of the gym so if you defeat some but not all of the defenders, they lose prestige points and at some point they'll lose a defender making it easier to take.
If you defeat all the defenders, they lose the gym and you can claim it for your team.
The highest prestige gym I've seen was level 7 or 8 I think, most seem to be 2-4 round here.
I can sometimes get at a pokestop from office depending where the gps centres on map
