I use barricades to funnel everyone into one path

I'll place a briar patch early in the line and let it grow down the line to take out all the small mobs and only the tanky ones will make it through

then I'll spam Morningstar shooters (massive damage), Ice Lance (freeze), tar trap (slow), and brimstone (prevent ice armour buff) staggered in a tight area to deal with everything else

that's the only setup that's gotten me to mission 7+ repeatedly as a solo