The most important stat to increase is your hit points. Make sure that it is always double your level. Without high hit points you won't get far in the battledome.

The other important ones are strength and defence however the boosts (damage multipliers which give you more attacking and defending power) aren't per level, rather they are in sets.

Check out this page for a list of the levels and the boosts at those respective levels: http://idb.finalhit.org/?searchtype=page&page=articles&a_id=80

So with strength and defence the way to train is to the highest boost that can be gained at your level and then wait. (Or you could just train up to double your level again.)

As for abilities the best ones are Sink and Burrow and I think Drain Life as well as one of the healing ones and a freezing one (Diamond Dust).
Have a look at this page for a list of all the abilities: http://idb.finalhit.org/?searchtype=page&page=faerieabilities

I don't know about the best weapons but it sounds like you've got all the good, cheap ones.