Is indeed a very good method not only for 50k but for the level 65 achievement as well.
Using the flail as well you can usually get;
Not in Kansa +150
and sometimes Gotcha +75
each, per kill, is between 850 - 1000sp per regular kill
Also helps to kill the bosses with different team ways, such as whiplash, touchdown, double penetration, heatwave etc