As I said it always depends on the preperation and hence the situation to a degree. Though that much Grunts might have also killed that Wapsmith and play in a different league.
Given that troop composition with an added tank, some preparation from a decently build party they would stand no chance. Though I am arguing here from the position of a Guerilla/Stealth warfare and no stand of where people charge each other on plain field. A regular PCs will not necessarily fail against one strong foe but the volume of attacks will carve through his doges/parries and sooner or later result in assured damage that drains you down. But in some urban or rural area with plenty of cover the trick is to stay hid d en, switch position and utilize hard and precise strikes. A sniper might kill all the special weapons and lay down suppression with his comrade from undetectable range and might switch position if needed. Explosives like Mines require preparation on the field yet a mortar allows constant striking from a save position. Add in pincer attacks from your other guys, the heavy use of stuff like smoke grenades and these grunts are teared to shred. The tank will only be of use if it has an actual target and is taken out so easily by several different tools.
We once had a similar situation and even with the assistance of some auspex regular grunts and at one occasion even marines were no match against our stealth specialists. War is all about bringing yourself in a superior position and utilizing all the tools you have at your disposal. That is how we play it and we rarely start engagements at close range below 100 metres as long as we do have the initiative - and that is the most valuable thing.