His passive is 1% for every 500 def, so at 5,000 def he's getting 10% heal. That would suggest this Islin has like 110k HP and 5k def. I'm confused why it is happening twice, and it seems to be healing him for more than the total amount - my champions do much more than 10-20k damage. 

Who? You are probably only one here using him 😆 

You know I didn't build yet :)

K is not using him.  I think he is fighting him.  My Islin is also still unfinished, but I have seen him heal like crazy the few times I have fought him in Arena.  Just take a bit longer to win.  

There has to be some sort of bug in arena vs him cause I just unloaded on him for 10 minutes with my full squad and his HP meter never dropped below full with no buffs or shields on him...he was indestructible as far as I could tell...so either someone was some how cheating or there is a bug

It not cheating and it's not a bug.

Protect The Troops (Passive)
Whenever this Champion is attacked, heals themselves by 1% for each 500 DEF this Champion has. Heals all other allies under [Ally Protection] buffs placed by this Champion by 30% of this skill’s initial heal. 

If he has 6k def and 100k hp, EVERY time he gets attacked he's gonna heal himself for 12k hp.

If your champs aren't hitting for more than 12k, against his 6k def, then he's gonna heal more than he gets damaged.

I think you're forgetting to factor in Masteries. He probably has Defense + Support trees. Support Tree is 5% more heals plus 10% more if less than 40% hp and 15% more if he's CC'd (Stun/fear/sleep/freeze). He is stunned so that's 15% bonus right there, not to mention Defense mastery increasing heal he receives

Second, either your champ has a multi-hit ability AND/OR has Phantom Touch AND/OR has Warmaster/Giantslayer, so that's 2 instances of damage which means 2 heal procs.

EDIT: Btw, I'd clip the screenshot better next time to hide unrelated stuff or potentially embarrassing things like using Edge as your browser 😁

He is excellent against Marius, aka annoying as f#$%...

He sucks. The problem with him is even if you have hard hitters your support champions usually have to attack once and a while. But especially with a Wukong/Islin team... use Ronda with her block passive. Use here to kill Wukong perminately and then use her to kill Islin. 

I didn't know there was a counter to Marius, that's good to know lol

I'm deleting that post :)