The reality is that a unit of Hormagaunts is not designed to take down a hyper elite unit like Terminators. If you change the scenario to be Genestealers vs Terminators, then things are altered dramatically in favor of the Genestealers (assuming they get the charge). Hormagaunts are line troops and line troops exist to take on other line troops. Hormagaunts on the charge will kill about 4 normal Marines, and lose about 3 of their own number, assuming 22 make combat as in your scenario. That's about as it should be.
To be fair Hormagaunts with furious charge, poison, and preferred enemy can easily wipe out 5 terminators vs 20 Hormagaunts. 45 attacks that hit with 33 wounds that equal 6 dead Terminators and 12 dead maries. Although you have to take upgrades which makes the model 10 points a piece with a hive tyrant near by giving off the preferred enemy aura. Even without the aura that still would be 35 hits with 25 wounds killing 4 terminators or 8 marines.
Noow if you look at no synapse, no upgrades than the Hormagaunts would do poorly. Although I would use Termagaunts with a Tyrant and Tervigon near by whicn would come out cheaper and produce much better results. I have had 20 termagaints wipe out 5 man terminator squads many times as long as I had my Tervigon and Tyrant near by.
