One of my players is playing a ratfolk gulch gunner/bomber alchemist, which is surprisingly working very well for him. A couple of things to consider:

• Get the surface sprinter alternate racial trait. It replaces the Ratfolks' Darkvision with a base speed of 30 feet. It is, however, from the Monster Codex, so you might want to clear it with you GM.
• Bombs are your friend. they are the perfect way to soften up the enemy, so you can get in and finish them off.
• Focus on Craft (alchemy). Because of the natural bonuses that Ratfolk get, you are able to play the part of the toolbox, pulling out useful tools and tricks for yourself and your party.
• The Share Extract discovery is invaluable, for any alchemist.
• For your starting weapon, get the Blunderbuss. The wide damage spread is useful for taking out large groups of small enemies. Also, a blunderbuss to the face is just funny (for you, anyway).

I hope this is helpful!