Against mech? Use the lord of change and/or 2 herald's of tzeentch to give the soulgrinders reroll to hit, with the prescience psychic power and give them the strength 10 ap1 attack.
Also, more screamers.
Could also be unusual and put the plaguebearers into a squad of 20, and use touch of rust (glance any vehicle on a 6) to wade in and smack up vehicles