Well you definitely won't want it to be shot by heavy warmachines, so keep it safe from them, and you don't want any sneaky fast cavalry and such trying to assassinate the priest, but it is better to kill them than hide from them since they are fast and usually fairly weak. It is also most effective near your battle line, so I would move it up behind your main line so it can cover with ward and the skink can cast, then when combat happens start throwing out burning alignment and add it to combat where needed. That is an ancient stegadon under there, so you don't want to waste its combat power even though the skink is fragile. If you aim it for flanks it should be safe, since even agains ASF things the impact hits will destroy most in btb contact. Or if you go for the front, see how much you kill with impact hits, if it is a few target the unit champ with the skinks or or maybe the steg to reduce attacks back at you.
Definitely no point completely hiding it, but at the same time you don't want to throw it away. It is a strange unit, I am yet to see if it is really effective or not.