What are the best ways of slaying Metal Dragons?

Depending on your level here are some gear options for Slaying Metal Dragons. 

Tips

Killing Metal Dragons in Brimhaven is faster than the ones in Kourend as the ones in Brimhaven have lower defence.

Staff of the dead

The staff of the dead is a one-handed staff that requires 75 Magic and 75 Attack to wield. It is dropped by K'ril Tsutsaroth, and counts as a Zamorakian item while in the God Wars Dungeon and in the desert Bandit camp. It is one of the few items that boosts the player's magic damage, boosting it by 15%.

Use fire wave (with tome of fire) or flames of zamorak for highest DPS.

Blowpipe

The toxic blowpipe is a two-handed dart weapon that requires 75 Ranged to wield. It is made by using a chisel on a tanzanite fang, requiring 53 Fletching. Players using the blowpipe have a 25% chance of inflicting venom on their opponent. However, if the opponent is an NPC and the serpentine helm is worn in conjunction with the blowpipe, there is a 100% chance of envenoming them.

Blowpipe has very high damage output in general and would probably be better against bronze. metal dragons are weak to magic though, so it's probably not great on iron/steel without really high ranged.

Toxic Trident / Trident of the swamp

The trident of the swamp is a trident-class magic weapon requiring 75 Magic to wield. It is made by using a magic fang on an uncharged trident of the seas with a chisel, requiring 59 Crafting to do so. Unlike its predecessor, it is only tradeable when uncharged.

Toxic trident is classed as an essential piece of PvM Gear and a good investment.

Best Magic Setup

your setup for magic would be:

• staff of the dead OR staff of light, casting fire wave
• tome of fire
• occult necklace
• slayer helm (i)

less important:

• ahrim's robetop + robeskirt
• any kind of magic boots (eternal best, doesn't matter too much though)
• seers ring (preferably (i))
• tormented bracelet, or your best magic gloves
• god cape (preferably imbued), or any cape if u dont have one

High Money Option / Unlimited Bank

• Dragon hunter crossbow (DHCB)
• Dragon Hunter Lance (DH Lance)

Sources

1. Source
2. Source