Arcane Strike provides a damage boost based on Caster Level. For characters that remain in their original spellcasting class, this is simple enough. But what if you multiclass Bard/Sorcerer?
You can use any caster level you like, but there is no rule about combining any separate caster levels, so you would pick the highest one. Bonuses to caster level, like an ioun stone, would count for this purpose. And a bard/druid can use the druid caster level (the requirement that you can cast arcane spells does not implicitly mean that you have to use the caster level of an arcane-spellcasting class; that would have to be spelled out in the feat’s benefit and it isn’t).