(note: Garchomp doesn't share a weakness with either Tyranitar or Excadrill - it still has difficulty beating things that beat those two, though)
I'd suggest Virizion as your potential partner. While it does lack a bit as far as physical defense (meaning you can't really switch it in on Earthquakes all the time), it's very fast and gets decent coverage with its moves. At the very least, it smacks the two types of Pokemon you want gone.
Virizion @ Life Orb/Leftovers
nature: Timid
evs: 252 SpA / 252 Spe / 4 HP
~ Giga Drain
~ Focus Blast
~ Hidden Power [Ice]
~ Calm Mind
If you can't RNG for Hidden Power or soft-reset or simply search the forums for one, then you might have to run something like Haxorus - it smacks everything really hard with its Outrage (even steel-types), though it is outsped by Salamence, Flygon, Garchomp,
[email protected] (though I don't think they're allowed but idk) and Hydreigon. You could possibly run a Choice Scarf Haxorus if you would want. However, Outrage hits so hard that it can 3HKO Ferrothorn counting Leech Seed recovery (with Choice Band) - which is an amazing feat.