Let burning coals fall upon them, or, "burning coals shall be thrown upon them." Let them be cast (or, "they shall be cast") into the fire, into deep pits, that they rise not up again. The clauses are declaratory rather than optative.
The psalmist sees the wrath of God poured out upon his enemies, who are at the same time God's enemies—they are cast into the fire prepared to receive the wicked—and plunged into deep pits whence they find it impossible to extricate themselves.