The game/gym battles broken asf?

So I was walking around and saw this gym holding 1 pokemon with CP 2000. It was weak to water so I thought easy win for me. I started the battle (was only that 1 pokemon in the gym when battle startet) suddently in the middle of the battle it couldn't lose health, suddently it had full HP suddently there was a blissey and chansey in the gym suddently there was two more strong pokemon (snorlax) and they kept getting full health and became unbeatable. Can someone explain? This seems really weird and yes I am fast at tapping in and out and so on so either it's hacking, laggy pogo game or just broken/buggy asf. I would have beaten that gym easily if it wasn't for health regen and suddently-more pokemon in the gym in the middle of a battle.

And when I try to do it it always says "you can't the gym is under attack" (something like that) when I try to health up my/teams pokemon in a gym or put one of mine in it.