Mortar Spectres are specialised enemy types, found only in Titanfall 2's Frontier Defense. Unlike the first game, in which there were many different variants of Spectre to fight, the Mortar Spectre is the only Spectre variant to be found in Titanfall 2's Frontier Defense.
Mortar Spectres are deployed in squads of four, and will endeavour to make their way to a location free of obstacles to deploy their equipment (usually a rooftop). Upon arrival, they will deploy Hard Cover and their Archer Heavy Rocket launchers, modified to fire in an arc (similar to the modified Quad Rocket employed by Mortar Titans). They will then proceed to bombard the Harvester, like their larger cousins.
Compared to Mortar Titans, Mortar Spectres are nowhere near as damaging per-team, but they can still be a serious nuisance if left to their own devices. If two or more fireteams set up (often at opposite ends of the map, just to further complicate things for the defenders) they can still inflict quite a bit of damage before being taken out. On the other hand they are also more dangerous to Pilots than Mortar Titans, as the team is equipped with anti-infantry weapons.
The Electric Smoke Grenade and Gravity Star are especially effective at countering Mortar Spectres, as either can be thrown into the center of the group whilst they're set up, and will immediately disrupt them and render them vulnerable to followup attack. They are also easily distracted, because a soon as you attack them they will all stop firing their Archers and will start firing at you.
Otherwise, explosive weapons like the EPG-1 and Softball, rapid-fire weapons like the Spitfire LMG, or long-range weapons like the D-2 Double Take can all be quite effective. A sniper can attain a high vantage point to shoot over any defenses they might have, whilst explosives or rapid fire weapon can be used in combination with a Gravity Star. Also, they take time to load up their rockets, so use this time to come and kill them