Be warned that they have a fairly long range pounce, and do good damage. Don't fight these unless you're truly prepared.

The best way to deal with a mantis is to use fire. Use flame throwers, fire wyverns, or oil jars.

When in a cave use oil jars and flaming arrows to set a fire trap for mantis,sabers,and other cave dwelling creatures

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