The most important thing in caves is a motion scanner. Dogs/rats have it as builtin action or you can make a few. Scanning allows you to see incoming enemies from beyond the walls, while being covered by said walls. Nuff said.
After scanning, use alt / alt+ctrl to highlight scanned dots. Scanners pinpoint enemies horizontally. If the enemy on different floor, it will still give a dot. You may then pinpoint the floor by bringing a rat or hybrid to the dot - they'll see the enemy above/below with their short psi-sense.
Scanners mostly trivialise many such missions - HQs, underwater, caves, ships, bases, etc. at the cost of some effort.
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Dogs are good for scanner alone, but they also have some anti-camo, some heat-vision (anti-smoke), decent night vision. Rats have all that increased.
Furthermore, dog's bark "damages" enemy TUs, thus 95% (5% is damn chupacabras) of incoming melee threats will be stopped by a decent full-TU dog. And they're pretty good in melee, being a sectoid's nightmare in right hands.
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Chtonites enjoy beating beaten into submission - synthsuits are very good for such missions if you have them. Silacoids are "weak" to HE, so use explosive rounds. I kinda like autocannon - AC-HE.
For Dim X outside, tactical grenade launcher / gas works wonders on spitters / megaworms / anthropods, but may need experience playing with smoke. Flys / whatever else is better dealt with HE rounds, IIRC.