This will be a guide for each of the wilderness bosses, the Chaos Elemental, Venenatis, Callisto, and Vet'ion. I will also briefly explain Galvek. I will be showing basic gear and items, all of which can be bought from the shopskeepers at ::shops. (But you should bring the best gear you're willing to use in the wilderness)
This should go without saying, but I don't wanna be responsible for it. People can and will try to kill you if they find you in the wilderness. If they kill you, they will take your stuff. Be sure you have a way to escape.
You have been warned.
First is Venenatis.
For Vene, anti-poisons and prayer potions are key to longer runs as she does drain prayer and just so happens to be a poisonous spider.
Venenatis is incredibly difficult if you don't come prepared, she has two attacks in particular to worry about, one of which appears as the weaken spell, it drains your prayer every time she uses it and she uses it quite a lot, so a decent amount of prayer potions or super restores are highly recommended. The other is a typeless attack, ignoring prayers, and hitting up to 56 damage. (Bring a lot of food)
During the fight, you can stand in the highlighted area next to the rocks to avoid her melee attacks, as she won't be able to get close enough to you, during which, you will want to activate the protect from magic prayer, or reflect magic curse, as the only damage she will deal, besides the typeless damage, throughout the fight. (You may need to lure her to the rocks by attacking her and running back) After doing this and keeping an eye on your health, Venenatis will go down.
Next will be Callisto.
For Callisto, you'll need a few anti-poisons, high prayer bonus, and a large amount of food, because he's a jerk like that.
Callisto primarily attacks with melee, so that is what you should be praying the entire fight, but he also has a few other attacks, one knocking you down, one slightly healing him, and two more that make this actually a boss fight. One of which is a ranged attack (I say ranged, but it is typeless), he uses if you are not within melee range of him, this attack will almost always land, will almost always do crazy amounts of damage (Max hit of 56), and the other is basically the same attack but he will occasionally do it while in melee range, still typeless, still unavoidable, and the only way to combat this attack is eating. Because of this attack, killing Callisto with range or mage is almost impossible. (Note: I'm not sure why, but at the start of the fight, unless you have a way to prevent poison, you will be poisoned)
The highlighted area is where you will be teleported to if you use the teleport tab shortcut, of which Callisto might be able to see and attack, so if you do use that teleport make sure you either run behind one of the trees to avoid his attacks, or show up ready to pummel his face in. Once the fight has started, be sure to activate protect from melee, or reflect melee, and from there the only thing you really need to focus on is his typeless attack. After all of this, within some time and effort, Callisto will go down.
For Vet'ion, prayer pots, run energy, a decent amount of food, and a lot of patience is key.
Being an undead, Vet'ion is weak to both crush weapons and the Salve amulet, so bringing both would really help the fight. He has three attacks, one being a typeless magic attack, the other two melee, one of which is also typeless. His typeless magic attack is used when you are out of his melee range, throwing 2 or 3 bolts in your direction and hitting the area around you, if you are within one square of any of these, you will take the damage they deal, but he doesn't make this attack while in melee range at all. (Note: This attack does stack damage) One of the melee attacks is just that, a basic melee attack that can be defended against using the melee protection prayer/curse, but the other deals typeless damage and is unavoidable if you are within melee range of him. Another important thing to note about this boss is that he has two health bars, once the first is depleted, he will turn from the regular purple Vet'ion, to a bright orange Vet'ion Reborn, effectively filling his health bar again.
Earlier, you read that you would need to have a lot of patience, that is very true. The easiest way to defeat him is by luring him to the orange highlighted area as he will be easier to attack from there, and using the line, highlighted by the yellow, to run back and forth to hit him between his attacks. That may sound complicated, but it is really simple. Sit out of range of his melee attacks by standing at the far right side of the highlighted yellow and wait for him to use his magic attack, the moment he does, run to him and hit him once, after that immediately return to the opposite side of the highlighted yellow, rinse and repeat until he is at about half health. Once he does hit half health, he will summon two skeletal hellhounds, both only attacking with melee so keeping the melee protection prayer/curse throughout the fight is mandatory. (Note: He will do this again during his second phase) While the hellhounds are alive Vet'ion will not take any damage, so killing them is the only way to advance the boss fight. Rinse and repeat, and with some patience, Vet'ion will go down.
AnD tHen The CHAos eleMenTal
For the Chaos Elemental, keeping your health up and keeping your inventory filled should be your main priorities, trust me on that second part.
The Chaos Elemental has three attacks, a rainbow colored splat that can do either melee, range, or mage damage and isn't typeless, a green pair of orbs that will remove equipment from your character, sending them to the inventory if the inventory isn't already full, and a dark red pair of orbs that will teleport you randomly in the surrounding area. (Note: Though it is possible to defend against its rainbow attack, there is no way to predict it)
The fight itself is fairly simple, you should use the absolute best gear you're willing to while in the wilderness and either pray the attack style that you have the lowest defence against, or use the curse Soul Split to assist in keeping your health up. From there, it's just a fight, keep the damage coming and the CHaoS ElemAnTal will go down.
For Galvek, anti-fire potions, dragonfire shield, or antifire shield, and some prayer pots is all you'll really need. Yeah, it's disappointing.
The only attack Galvek uses is a dragonfire breath attack, easily defended against using the magic protection prayer/curse and an antifire potion. Easy and simple.
The only thing to worry about during the fight is other players, as Galvek is almost always a hotspot for pkers, but other than that there is no real danger if you've prepared correctly. (Note: A high prayer bonus would help the fight)
This is my first guide, so I'm not expecting it to be perfect, but I hope it helps everyone kill these bosses a bit more.