Definitely CR 5, I think.
So we just have the flavour text to do? How about this:
A living cone of congealed fire, roughly the height and breadth of a man, with half a dozen strands of solid flame writhing from its sides. A single burning eye glares from above its hungry maw.
Residents of the Elemental Plane of Fire, Flame Ropers are similar in appearance to the ropers of the Prime Material, to whom they may be related, but are smaller and have bodies made of fire and burning materials. While the ordinary roper lives in subterranean realms, the flame roper prefers to dwell in areas of "open fire" where prey is plentiful, they are most common in jungles of fire trees, and plains of burning coal and flame grass.
Flame ropers can hide among a wider range of terrain than material plane ropers. They can camouflage themselves as a fiery plant, or as a natural feature such as a pillar of flame, a small mound of burning coals, or a red-hot volcanic boulder.
These brutish creatures barely rate as sapient beings, but can be taught to serve as guardians by more powerful fiery creatures. For example, some efreet nobles use flame ropers as "ornamental plants" in their gardens.
A flame roper stands 5 feet tall and tapers from 1 to 2 feet in diameter at the base to 6 inches across at the top. It weighs x pounds.
Flame Ropers cannot speak.
Combat
Flame ropers hunt like regular ropers, camouflaging themselves and waiting for something to wander within range of their weakness-causing strands, then try to snare their victims and drag them in to be devoured.
Flame ropers are not bright enough to negotiate if they face defeat and are too slow to run away, so usually fight to the death.