I second Sethis, treat them like super tactical since there special ammo can allow them to deal with pretty much any none vehicle model. BUT and this is important, Sterngaurd do not have a boolet to take on 2+ armor save targets, so I would run them with 8 bolters, and either two plasma or two grav guns to deal with termys.
I favor grav personally since I don't like the chance of cooking my own guy, plus if you stand still your pumping out a an extra shot, but compared to plasma you lose range if you move for the same number of shots as a rapid fire plasma gun and your lose something in anti armor, though personally if your in a situation where a plasma gun is your best bet to take out a vehicle that needs to die badly enough to waste the anti infantry power of the guard then you may be unsalvageable.
note: if you see me giving tactical advice: just assume i have a internet tab open to 1d4 chan and I'm summarizing off of that.
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