Oh yeah: if you haven't already, always get at least level 2 doors ASAP when heading into a Mantis sector. Almost every one of those mean motherfuckers carries a teleporter, and you do not want to get up close and personal with them.

If you've got a medbay upgrade (which I like because it offers many 'good' choices to events) then you can easily defend from Mantis boarders, just make sure that you fight in your medbay where you get healed all the time, they'll teleport out when they're almost dead, though - try to disable their teleporter for that moment if you can.