Several ways, I think.
1. Imperial Guard Specific, but I think a basilisk barrage on the IC in question holding it would put a damper on his parade; assuming, of course, that he was targetable for whatever reason.
First off all, I ussually have 2 portals. 2nd of all, I give them sometimes to Sybarites in ordinary troopsquads, so you guess which one it is. Or I give it to Heamonculus, which you ussually don't take down quite that easily. 3rd of all, take into acount the cover on the table, the DE player having 1st turn and there will ALWAYS be a portal on the table (if you don't have some noob DE player as opponent)
2. 1st turn deepstrike attacks. Deep-striking a deathwing termie squad, some Tzneetchen flamers or whatever on it should at the very least give you a good shot.
Yes, please... put some valueble units close by. I ussually get CC units out and once they come out they'll get into CC, so you basically won't have time to shoot...
3. If you're fast enough with a big enough thing, drive a honking big vehicle over it. Logic is they can't be within 1 inch of an enemy unit until assault phase. Therefore, you put a big vehicle over it, and for lack of deployment space they just die instead.
Just how I'd deal with it, my 2 cents.
Rules specifically say that the portal is impassible terrain. So you can't drive a tank or anything on it. You could however try to surround it with a squad keeping 2" coherency. Then indeed the '1 inch of an enemy unit' aplies.
So as an experienced DE player who uses WWP all the time I'd say option 3 (with my modification) gives you the best shot at countering a WWP attack. Make the DE player think he's so smart and so fast and let him place the WWP as deep into your zone as possible. Once the portal is placed surround it with a squad jump infantry which you kept waiting (they move 12") and shoot/assault the 'portalplacers' with some support unit that was waiting too (perferably jumpinfantry too). I've noticed that if SM has more squads that move 12" you'll stand a better chance against DE. Else DE decides exactly what is going to happen with their mobility.