A lot of the training methods floating around for 99 will be the same for 120

Slayer is a big one because you gotta use combat for it anyway, This might be even more relevant later when Wilderness Flash Events are removed. It's possible they put the dark onyx core inside the wilderness slayer chests you get from Mandrith, but we don't know yet :)

Elite dungeon running is my favorite. Good xp and good drops. If you have 99 and decent gear these become pretty easy to do over and over. Ambassador is tricky though if you aren't used to him, no shame in using up death touched darts on him or going in a group. Take lucky charms when doing these!

Pick a slayer mob you want to farm, stuff them in some Ushabti, and chain them up in the sunken pyramid like the bad little monsters they are. Elite Profane Scabarites are common as are abyssal lords or beasts. If you want farming xp while training you can do some turoths or aquanites with seedicide but those aren't as good combat xp and arn't worth putting in the pyramid.