Since we were discussing this in that FearFactor topic, I decided to take this seriously and present solid evidence about this Boss. See for yourself:
Regardless of what attack u use, if u get up close, it will hit u hard as hell. Ranged he will run to a certain position, but we all know u can't kill it ranged with magic. Melees are the ones that actually kill Isiloon and if they aren't gettting hit and just taking it with pots then its pretty clear this is bug abussing. The more ppl attacking it, the less frequent it will hit the same target, still it will always hit really hard at melee range.
That theory about hitting with Raptor at max range fell apart quite fast.