Timid is the best Nature for Froslass overall. However, it generally has more options than just attacking...
...you see, whilst Froslass does get a very nice offensive movepool, it suffers from one problem: Base 80 on both offensive stats. Three attacks and Destiny Bond can work, but really, you need something more...
Here's a few things of note about Froslass:
- Froslass is the only Spiker in the game that is immune to Rapid Spin - this certainly makes Froslass quite unique. Spikes is definitely a notable option to consider for it for this reason, although setting up the Spikes can be difficult given Froslass's somewhat mediocre defense stats - Base 70 on HP and both defenses is the same as Lucario, and Lucario certainly isn't known for its defensive prowess.
- Snow Cloak can be infuriating to deal with if you can set up a Hailstorm, and STAB, never-miss Blizzard would give Froslass a very nice boost to its otherwise lacking power. Although it can be a bit tough to set it up, Froslass doing standard sweeping tends to be underwhelming otherwise, especially since a very similar Pokemon to Froslass can run a 3-Special Attack + Destiny Bond set with even more effectiveness: Gengar.
- Other interesting moves for Froslass include Thunder Wave, Confuse Ray and Light Screen. Substitute works very well combined with Froslass's high Speed, and Sub + Snow Cloak effect can get very nasty in a hurry (*cough*Brightpowder*cough*.) Generally, you need to make the most out of Froslass's other options to avoid being outdone by your fellow Ghost-types.