I'm currently chaining for Luxio and I was wondering if it was possible for me to find a Level 34 rather than a 24 ("!" on the dexnav)
You just have to raise the encounter level (the number of times you encountered luxio in your case). Then, you will have a higher chance of finding it.
Source 1: Experience
Source 2: http://bulbapedia.bulbagarden.net/wiki/DexNav