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).
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