Updated: Oct 27, 2018
In order to kill Dagannoth Rex, I'd recommend having at least 70 in all of the combat stats, which is pretty low for a boss monster. You get Dagannoth as a Slayer Task very commonly, and if you have one as a slayer task, you can make a heap of profit by doing the whole thing at the Dagannoth Rex. You can use the Slayer helmet if you're on a slayer task, but at the moment, the full Guthan set is only about 3 mil.
So, this makes it a really, really good set up to use at Dagannoth Rex right now for low levels, and it's a lot cheaper than other set ups that use the Serpentine Helm, or the Verac's Helm, or the plate skirt. For those that don't know, Rex has a magic level of 0. Which means that you can wear any armour with any Magic bonus, and still be able to hit him the same as if you wore Magic gear. The best weapon to use at Rex is a Trident, and if you don't have a lot of money, you can buy an uncharged Trident at the Grand Exchange for about 50k, and then charge it with 200-300 charges just for one trip at Rex.
The two things you need to focus on are Magic damage boost, so the Occult necklace, and Prayer bonus, as this is very important at Rex as well, especially if you have a low defence level. In your inventory you do need Rune Thrownaxes if you're going on your own. That's the only way to get into the Dagannoth Kings.
Also, if you're going on your own, you will need a pet rock and you can drop that on the gate as you're trying to go in. It will work as if someone else is standing there.
I brought a lot of prayer potions, and that's for players that have a very low defence level. You'll see soon when I go to kill Rex, that I don't even need to use Prayer there, and that's since my Defence and Magic level are very close to 99. But, for those people that have around 70 in those stats, it is recommended to Pray Magic while you're killing Rex. Navigating through the Water Birth Island Dungeon isn't too difficult. But, you just have to make sure you get your Prayer switches right as you go through. Once you get all the way to the bottom, there's a root that you can jump over to peek into the Dagannoth cave in a safe area. That's really helpful for world hopping, and if you need to find an empty world.
The hardest part about Dagannoth Rex is getting into the dungeon. A strategy that I use is that I like to spam click on the ladder so I can peek in there really quickly to see where all the Dagannoth Kings are, and then pop right back up without taking any damage from any of the Kings. As you're entering, you should have protect from magic on. When going in, do not be aggressive to both Supreme and Dagannoth Prime. There is a good chance that they'll attack you a few times, but then eventually they'll walk to the other side of the room. That allows you to hug the wall, and run South, hugging the coastline all the way to the Eastern side of the room. If you hug the Eastern coastline, the other Dagannoths will not attack you. The only one that's in reach is Rex. Its a good idea to use long range on your Trident, and what you have to do is attack Rex, run over to the coastline, all the way to the East, and wait till Rex comes a little bit closer, and then run directly South. You should put on Protect form Melee if it's your first few times doing this, because there's a good chance that he'll hit you. But, once you get better and better, you'll be able to lure him into the safe spot so easily.
The fight is very, very simple. You won't take any damage, except form the things around the outside on the coastline, and then once you finish your kill, you can switch over to your Guthan's War Spear to heal up on the guys attacking you, and that basically means that you don't need any food with you for the rest of the trip.
As you can see, with me not using Prayer, I'm literally not taking damage, and I'm not using any Prayer points. So, this means I can stay here pretty much endlessly as long as I have enough Antidote Potions, and it makes it really handy for Dagannoth Slayer task, because there isn't much banking involved. You can just camp here. You can use Blood Barrage to heal up if you don't want to bring Guthan's, but that has a higher requirement, and is more expensive to use than Guthan.
Rex drops the Berserker ring at a rate of 1 in 128, which is a really good drop rate for a 3mil drop. He also drops the Warrior ring at the same drop rate, but that one's only 80k. Depending on how many kills you get an hour, you can get approximately 700k - 1mil+ per hour killing Dagannoth Rex. The spawn time is a little bit long for Rex, so you can't get a huge amount of kills for Rex. But, 30 kills is still doable in an hour, and you also have a pretty good chance of getting a Clue Scroll: 1 in 42 for a hard clue, and 1 in 750 for an Elite, and 1 in 5000 drop chance for the Pet Dagannoth Rex.