Minecraft wiki entry for Defeating_a_pillager_outpost, associated text: You can pour lava down where they are standing, melting them all. The lava destroys the pillager drops and pillagers have good enough AI to avoid lava.
If playing in Java Edition and have a sword with Sweeping Edge, you can go close the group and damage large amounts of pillagers at once with your sword, raising your shield while your weapon recharges. When the pillagers are about to fire raise your shield to ricochet the arrows back at them. This can work in Bedrock, but not as well since there is no sweeping edge, but as a benefit, there is no recharge.
You can use a crossbow with piercing arrows or multishot fireworks to hurt and kill several pillagers at a time
You can lure a Creeper or use TNT to blow up the pillagers once they are together. Multiple blocks of TNT can be more effective but can also destroy the pillager drops.