Walk the army to the gate, show off the giant monsters, maybe fly the dragon-snakes overhead, and tell the townspeople to surrender or be destroyed. Seeing as the goal is occupation, not destruction, getting there without fighting should be the preferable option. If the townsfolk have any common sense, they'll take the surrender. Trained or not, those are commoner we're talking about, and even if they are defending their homes and families, they should realize the odds.
Static defenses (and terrain, which wasn't mentioned) are a good force multiplier, but the lizards are numerous enough they don't have to concentrate all forces into a single spot, and spreading the defenders out leaves them vulnerable to overwhelming force. Sure, the defenders can shoot two goristros to death if there's enough of them together, but can they do it in 5 places at once, while the goristros are supported by other monsters, while being bombarded by giants (Storm Giant's lightning strike safely outranges simple weapons, by the way) and magic, with fliers causing havoc inside the town (you can even have the archmages ride the fliers for some extra oomph) while the main force keeps defenders too busy to respond, and there's a (giant-created) storm raging?