Get familiar with the Castellum cause it is the go to place to complete most catalysts, as for the Polaris Lance specifically just stand back and let them come to you and keep firing your burst round when you see a few of them together. What I did was shoot each cabal a few times to lower its health so there were a bunch with low health and use the burst round to melt the rest of the health and it took maybe 45 minutes to complete the objective.