I was wondering if I took only one level in druid (mainly for natures bond) and never took a level in it again does my animal companion level with me, or does it stay at first level?
Basic rule is that the stats for the AC are based on your Druid level, not total character level.
The only ways I can think of to get an Animal Companion that levels with you independent of class combination are either 4 levels of Cavalier and the Horselord feat or the Nature Soul -> Animal Ally -> Boon Companion feat chain.
Many Classes have a way of getting an animal companion though through archetypes, bloodlines, domains etc: Barbarians, Paladins, Oracles, Sorcerers, Clerics, as well as the obvious druid and Ranger.
