Snipers launch stun attacks after every 3 hits. Once your Guard gets stunned, enemies can easily get through. To prevent this, use your last placement to assign a Ranged character on the tile highlighted above.
Take advantage of the rule "last placed characters are priority targets of enemies". Use Summon Operators like Deepcolor, Mayer, and Magallan to take advantage of this!
Bring At Least 3 Melee Operators
Enemies come from 4 camps in this mission. However, they only use 3 lanes to cross to your camp. Place your Operators at the center first, right second, then left last. The final lane is used by the Sniper and you can deal with this with a range Operator of your own.