I think the freeze effect looks easier. Make good use of motion vector tracking to track on some icy textures and you could try some ML human body matting to get a cool grade on them. I have found some really good ice elements where a solid black frame will begin to ice up. Would work really well for the reveal. You can be more specific with tracking in some icicles. I would be using some VFX mist elements from a stock footage site.
Fire feels like it would be a little trickier. Depending on the movement it can be hard to get stock footage to look like it is tracking to a subject. You might get away with it
Not sure but this might be possible. Camera analysis and export rough geo with camera.
Use this
Import back in then flame away. As @PlaceYourBetts says, you need the stuff to track. I think it would worthwhile shooting a moving camera too. Rotating round the body. Best to do some tests. I know it’s going outside of flame but I reckon you could do it yourself for minimal cost.
You can totally create the fire effect on Flame using stock elements of fire. I have done this several times with good results. It is easier if the subject doest move that much.
If ActionVFX assets don’t pan out in Flame, and Embergen is out of reach, you could find a Maya/3DsMax artist to help out. Chaos Phoenix/VRay has sims for both fire and ice.
We’ve just done a few projects where we combined Cine4D/3DsMax with Flame/Nuke. Build some stand-in geo, track in Flame/Syntheyes, take into 3D app for texture and render, then comp in Flame. Works great.
Also, if the project has not been shot, it helps to add some interactive lights during the shot. This is to enhance the effect once you comp in the fire elements.