The problem is, the opponent goes,
"Hmmm, I see a Galvantula. Will I switch in a pokemon that will easily beat it, or will I send in a pokemon that is as good as dead against Galvantula?"
If your opponent is the least bit sane, Spider Web won't help much.
I think he meant that after they send in their galvantula counter, you call back your galvantula and send in a counter to their pokemon.