The Dragons from HTTYD were already trained!
In the first movie, Gobber states that 'dragons will always go for the kill', which is debatable because of the bias against dragons, but also because we see the dragons in the training arena NOT go for the kill' several times.
1- Meatlug- while yes she was shooting lava blasts at the kids, but all of her shots hit their shields. Especially when Fishlegs raised his shield above his head, she could have definitely hit him if she wanted to. And finally when she had Hiccup pinned against the wall, she paused. Granted it could be the director making the moment dramatic, but the only in universe explanation would be that she didn't want to hit him, and could hear Gobber coming to Hiccup's rescue. And speaking of Gobber, you're going to tell me that she didn't even take a shot at the one person in the arena with no shield?
2- Stormfly- this girl honestly looks like she's having a good time. Again she's taking very close shots, but she also doesn't seem to be taking it seriously. She even takes a moment to laugh at Snotlout's bad throw.
3- Barf and Belch- these two (one?) didn't even attack with teeth or claws. They let out some gas and sparks, but never actually ignited it. The only aggression they showed was knocking the kids down and getting scared of the eel.
4- the terrible terror was just a menace, but hey, tiny is allowed to be angry.
Point is, none of these dragons 'went for the kill' but why? And how does this tie into them being already "trained"?
We don't know how long those 4 were held captive. Had they been there for years? Had they seen/heard what happens when a dragon does maul a teenager to death? Training to fight dragons is dangerous, but you can't expect a Viking parent just to let the dragon that killed their kid live.
So the dragons were subconsciously trained.
Fight, act aggressive, but don't kill or you will be killed. Give the kids enough practice and the rest of the time you'll be left alone.
