Shin Okada
Explorer
If you are in animal form, they probably shouldn't know you are casting. Even if they do (e.g. after the first time), you can fly.
By RAW, opponents with spellcraft skill can still understand which spell are you casting, by watching/hearing your modified Somatic/Verbal components.
At level 15th, flying is quite a popular ability and not particularly a good defense. And You are staying in one place while casting a Summon spell anyway.
On top of that, those doesn't matter when your entire party is caught within the area of spells or breath weapons.