If you are performing the bosses you'll require a whole lot better armor. Look at the God Wars Dungeon guide on the site for more info on that. If you go in with just complete rune, it will not take you long to RS gold get slaughtered. You will need barrows/god armor (The type you get from the dungeon, like Bandos) a LOT of meals, at LEAST 80+ ALL combat stats and 70+ Prayer will make your life a whole lot simpler.
In summary: Get more moneyup and down your combat stats . I'd recommend you get each battle level up one level first, then try and get Prayer to 60, bare minimal. Consider doing it via slayer for some decent money, which you actually need. Rune is fine. Barrows is better but rune is good if you do not have the money to spend. You can even range him. 'Slaughtered' is overreacting. If you go with a group of 4/5 you can last maybe 10 kills rune. Likely 15 with Barrows, it doesn't really make that much of difference.
Rune is cheap and easy to replace, since there's a constant supply of this. You don't need Barrows. He won't require a lot of meals either. A complete devise of Tuna potatoes would be good. You're doing everything by the book. I went like 75 all fight stats. It took me a while to find a group, and paradoxically I was the 2nd highest level in that group, and we managed to get around 10 kills . Completely off topic to the query, but stop banging on about Slayer. He's 42 Slayer. What is he going to cheap OSRS gold kill to create'decent money' there? Terror Dogs for Granite Helms? But if you are so adamant that Rune is a pile of shizzle then how's he going to manage that?