I'd say around lvl 90+ for metal dragons, they have high defense and re extremely tough for lower lvls. Take prayers, anti fires, full initiative and dragon mace/Verac's flail (+5/6 prayer bonus) and you'll last for ages. Me and my friend did a test on how long we could last wearing the above and we lasted for about 2 hours on 1 potion and full prayer. Don't forget to wear a holy/unholy symbol rather than amulet of glort. As unholy/holy symbol gives +8 prayer bonus!
For barrows I'd say at least high enough to use fire blast (lvl 59). With about 70 defence and you'll do fine.
For legends, just have the requirements (45 herblore, 50 agilty etc). Also have around 70-80 combat and you'll breeze through it.
Lol opal tiger, you get my posting up so much. Jking
~Demon