When the island spawns it creates like it's little own sub-server area. Once you cross the server line, about 1-2 screens around the island, you cannot recall or gate. This is similar to any champion spawn area, even Ilshenar & Tokuno, where you can't recall out of the champ spawn areas while the spawn is active. You can hop back on your boat, sail 1-2 screens away from the island (it's obvious when you cross the server line) and recall to the boat. I believe this is intended, it's not a private peerless instance, but set up so anyone can find it once the island is spawned.
As far as beating Corgul, he's humanoid so repond works well on him. It's a little challenging because he's a necro user, and some of the island guardians are undead, so if you're holding a repond slayer you'll take double damage from these folks. Also, it is NOT necessary to kill the guardians before taking out Corgul himself.
We were able to kill him successfully a number of times with a couple tamers and a couple archers, skilled folks. We also arrived at the island once when a group of 5-6, shall we say, non-skilled folks were there, and they were just dying left and right and hadn't scratched him one bit. Discord is definitely a plus because his resists are 90s pretty much all around.
Regarding soloing, I've attempted to solo pretty much everything in the game, and as of this moment, I'll say it's probably not possible (I can't think of a way). I haven't tried a sampire on him yet (unfortunately I haven't gotten the booster for my account that has my sampire). I will give this warning, I fought Scalis on my sampire and he has anti-samipre measures. When I hit him, I got a message (not exact) "you take damage from the corrupted life force" or something like that, so instead of leeching life I took more damage from leeching. I know this discussion is about Corgul, but I wouldn't be surprised if he had something designed specifically to prevent sampires from soloing.