So when FFX first came out I loved it but I hated the Sinspawn gui boss. I think I only beat it on the original version purely by chance and now 10 years on I'm stuck once again in the HD remaster. Any FF masters out there with any advice? It has a tendency to kill my aeons in 2-3 hits and from then on I'm simply trend water until everyone dies...

It's been a while since I've played the game, but watching the Quick Look where Vinny fought it reminded me of that fight.

A good strategy is to go into it with all your aeon's overdrive meters filled. At the start of the fight, summon an aeon and overdrive. This will kill off the arms, leaving the rest open for attack. Attack the head until Gui knocks out your aeon, then keep attacking the head with Wakka and Lulu until the arms respawn.

Summon your next aeon and repeat. Once the head is down, just keep killing the arms (if Auron is a decent level, he may be able to do it in one swipe (probably a good idea to Haste on him too), then unleash your most powerful attacks and character overdrives on the body. Keep everyone healed, kill the arms as quickly as possible whenever they respawn, and you should be able to whittle it down.

The easiest way I always deal with that boss is to just have Lulu learn at least one of the -ra level elemental spells before I get there.

Ok, thanks l'll give those ideas a go, really hope I push through this time.

Just beat him last night, and as @marokai mentioned, I had Lulu sorted out with Fira as my main heavy hitter. Auron does good damage with Fire sword as well, and I used Haste wherever possible. I rolled into the fight with 5 of my 6 characters ready to overdrive, so once I killed the hands for the first time I unleashed overdrives. Concentrate on killing that head after killing the hands for the first time, once the head it out of your way everything becomes much easier. My summons didn't help much as Ifrit went down in 3 successive hits, don't worry if you loose them in the fight, the party primed with overdrives will be enough. You can also use Yuna's Nulshock on your party to stop damage from incoming thunder attacks.

Using a summon in overdrive can quickly eliminate the head, but barring that, use haste on Auron with Tidus (or in my game, VD69YOLO, because I'm apparently a twelve year old frat boy in training), attack the arms, and keep Yuna healing. If Lulu is powerful, you can use her in place of Auron, but I like him for his high HP. If Tidus has a spare round, have him caste haste on Yuna too. Having her able to heal everone quickly of status ailments will definitely help.