HQ assault in '98? You're pretty fast. No need to hurry that much, in my opinion.
Anyway, in addition to what the others have said already, you can also use some of the bulkier melee weapons as can openers, if you're desperate. Something like the bardiche maybe, if you have found that already. Flame glove and Staff of Dagon also work, if you have found and researched them.
I tend to use any sort of rocket launchers/RPGs, that I have found in missions and kept just for such occasions. High-Ex is always a winner, but you can also use dynamite, which is weaker, but at least you should be able to buy it at this point.
Other options work well but tend to take longer. No problem, if you have good cover, though. Fire and miniguns deal little damage against armoured cars but reliably due to ignoring/damaging armour, respectively.
It is best to deal with the armoured cars and any other enemies on the surface as fast as possible, before too many troops pour out of the underground base.
Again, no need to hurry. Take your time to stock up on weapons, if you feel you have to. As long as there is other research to do in the meantime, you're not losing out on anything.
If you have the money to spare, I would also suggest buying an Osprey, just for assaulting HQs.