Plant six reality anchors (two of each type) upon the beach near the barricade. (You can place them anywhere in the ROI, but the closer they are to the barricade, the less chance there is that you'll Tab-target them.)
Head back to talk to Veris. (If someone initiates a wave during or immediately after step 1, you may need to wait until the wave is over before Veris will update the quest.)
Kill 50 Horde mobs. Groups of Skirth slashers will spawn in the water and race up the beach towards you. Stay near your traps - they'll damage and weaken the monsters.