@Lee118
Not a nice experience for you to go throught - here are a few thoughts and suggestions for going forward.
You don't have a security deposit, which is a big mistake. Even if difficult to claim against, it sends a message that there will be consequences in the event of damage. Add one immediately - it will only put off those intent on partying - and remember guests do not pay it up front in any case.
Without a security deposit, you are then really at the mercy of the guest as to whether or not they will reimburse you.
You also accepted a booking from a guest who was trying to pay outside the system - this should really have been a red flag.
You talk about excessive damage, but only mention the flat smelling of smoke. You need to at least get professional quotes for deep cleaning of the flat and repair of the excessive damage, Without knowing what the excessive damage was it is hard to know if you will be covered by one of AIrBnB's insurance policies. You should also ensure you have your own comprehensive insruance in place. Get in touch with AirBnB and make sure you have photos, quotes for repairs, etc...to have any chance of success.
Finally - review your guest, but keep it short and professional, along the lines of: I cannot recommend this guest and his/her party as they caused significant damange to my property while holding a party.
Good luck and remember it is a rare occurence - not that this helps when it happens to you.