The mantis has two attacks, the close range hack and slash, as well as a devastating pounce. The pounce attack is long range and does considerably more damage than the hack and slash. Mantis aggro from a considerable distance (as in, if your on foot get out of there fast) so the pounce attack is very deadly. However, fire damage weapons i.e. Wyvern fire, flamethrower etc. do more damage than other rounds, the same goes for other insect/arachnid creatures
They drop organic polymer which can be used for ghille armor. Ran into 2 as I was leaving a cave and barely killed them using a pike. Dont ever let your guard down in caves, even when leaving and it was empty before
Chances are, if you see one, and it's not from a large distance, it already is going to try and murder you. They have an immense aggro range, and they deal a huge amount of melee damage. Unless you're very well armoured, I would suggest staying away. If you want to kill one, however, I would suggest using whatever tool/tame that you have that gets organic polymer the best, as they are the only creatures that can give you it (other than Coelaocanth from fishing), and it is useable for Ghille armor (which is almost undoubtedly the best armor in Scorch, as it halves aggro ranges and has even more hyperthermic resistance than desert cloth armor, with the same defense rating).
If your in foot you may want to run unless you have the fear to kill one of these. There strongest attack it when they jump at you. I've found it best to have any animal that has a high attack around when battling one of these.
If you have a high level dire, sabre or any other high melee dino which can be picked up just carry them to some mantis (pref saber with saddle 4 defence) drop them then let the mantis hit you. You clean up while the sabre malls it.