The problem with this particular Genestealer player is that there's nothing to screen against since he likes to keep nearly half his units (especially the Genestealers) in reserve. By the time they show up on the table they're well within charging distance. The best I was able to do in my last game was spend the command points for Auspex scan when they showed up, but even that doesn't thin them out too much. But you're right about the Vanguard setup I was initially suggesting, that wouldn't do much against the large number of models, more suited to bashing a tank or something. Which is something I'm still interested in being able to do, lol. But I now realize that I'm trying to solve two different problems I face on the tabletop with this single unit, which I now see just isn't feasible.
The trick to killing genestealers is to just put as many shots as you can into them. Thunderhammers arnt going to make them any easier to kill. Actually it will make them harder to kill because you’ll be at -1 to hit.
Always have a screening unit up front. Before they get within charging distance you’ll need to push your screen in close, that way he can’t move past them. Put as many shots into them as you can. If you can get them below 10 models they go from 4 attacks to 3. That means 10 genestealers get 40 attacks, and 9 get 27 attacks.
If your screening with intercessors, you might be better off charging him to get him below 10 models. 5 intercessors can actually do 11 attacks hitting on 3’s and wounding on 4’s in close combat, which is one more attack than if they shoot. The -1 ap of their shots doesn’t matter because of lightning reflexes (there might be a hive trait that gives them a bonus though.)
That’s one more attack then if you wait for overwatch and you only hit on 6’s there. It was worth it for me but I was also trying to keep him from getting any closer, and it worked.
Ps, if your against Primaris, sternguard basicly do the same thing but with less wounds.
Don't count them out of the running there. That 3rd place list from FMGT was built to do both jobs very well. It's all about size in that case.