Can anyone give me a brief list on which renowns are considered the hardest and easiest to take down?
i assume the wyvern is the hardest and the fire elemental the easiest?
If you are doing the gobln spawn with a warrior, you can use a goblin slayer talisman to make things easier for yourself. These are the reward for the Neville Brightwhistle escort quest in the Underworld.
The Fire Elemental isn't too hard, but sometimes the Fire Demons can catch you with a good uppercut. There is a mix of elementals and demons on that spawn too which means you have to be on your guard with regards to your weapon as Ele/Demon are opposing slayer groups.
The Skeletal Dragon doesn't have much of a preceeding spawn, you only need to beat on some boneheads and two skeletal liches to make it spawn. However watch out for any patchwork skeletons lurking around as they will knock you off your mount given half a chance.
There are a few annoying red gargoyle spawns around the Abyss now, they can sometimes mess up your efforts to defeat the champ more than the champ spawn will.