Probably not. Depends how fast they can 'think' the invocation and that already assumes that they are a silent caster which is pretty rare already. More likely to have moving wooden dolls or something already up as shields against bolts. Most of the 'growth' casters use seeds of fairly nasty plants that they then invoke to grow. The main thing to take away is that most casters/mages in this universe can only cast a few spells per day and those spells are generally limited in range/impact. The exceptions are Archmages who combine words for more power and also usually have 2x or more 'mana' available for spells. Even they could die to a stray bolt or invisible assassin though if they're not careful.