How will you deal with Declarations and Fate Point Invoke for Effect?

As far as permanent aspects, you can only have the seven (High Concept, Trouble, Background, Rising Conflict, Story, and two Guest Stars), though those can be changed at any milestone. You can however have any number of temporary aspects. Those are the aspects that you create through maneuvers and declarations, as well as consequences. Also you can make use of any aspect that you are aware of. Those would be aspects on the scene, other characters, or even the city.

You could use Thaumaturgy to put an aspect on you that has a duration that is so long (years) that it becomes more or less permanent. Is this really usefull? Maybe, but you can do it.

I think it's okay. More than a +2 or a bit of declaration isn't going to come from it. Where are you fearing does this become a problem?

So, in the game, you can get up to some pretty impressive levels of damage--powerful weapons, strength powers, spellcraft in general, so that any given character could theoretically be attacking and dodging at 5 (or even 6 or higher), and doing 5 stress or more each shot.

This, for me at least, makes it something of a challenge to create meaningful adversity in battle without resorting to giving the bad guys Toughness or Speed powers to compensate (I mean, even in the books, a handful of mortal goons can give anyone a little bit of trouble).

So what are some of the ways you guys even the odds against your PCs from time to time?

Hit them hard. HARD!
It's not so easy to get killed in DF so make them bleed and make them suffer. Let them spend extreme and severe consequences and enforce them hard later. Compel the shit out of the outcome of the battle. Don't fight to let them win in the end.

You seriously have no problem with someone being able to change core concepts of your character with a few dice rolls?

Lets say you are playing a Wizard of the White Council.  You also happen to be a strong advocate of PETA.  You also think chivalry is not dead.

In a few games (or in theory a few social combats in one game) a social combat expert now has you convinced breaking laws of magic is fine.  PETA in fact stands for People Eating Tasty Animals...or at least it does afte a few rapport rolls.  Also, now chivalry is dead, your girlfriend is sorta pissed.

This is way more powerful than bluff checks and sense motive checks or diplomacy checks in the D20 system. 

I think roleplaying over a large period of time should be able to slowly shape another character's thought processes and behavior.  I think events in games should change characters outlook on life.  Certainly not a few rolls or a social combat.

That is my problem with it.

In all of this cases i can simply walk away from the argument if i don't want to risk it. It also gives social characters a lot of power, something that normaly is the tuf of combat monsters.

Have a look at Burning Wheels Duel of Wits to see where i am coming from.

I am kind of the opinion it is never a good idea to force a player to take law breaker, considering even without becoming an NPC lawbreaking represent a massive character defining change that really shouldn't be forced on a player.

You should always give them the choice but the concept should be on the table at all time.
And i still think it is okay to compel a "low discipline" type aspect to say "if you attack these mortal goons with magic they might die so don't attack them".

It's most fun with a strengh power and completly oversized blades. Think tiny the gruff.

Which is bad game design, and creates false dichotomies during character creation. One character type should not be able to edge out the rest. It's fine to have specialties, but wizards don't have specialties - they're amazing at everything between thaumaturgy and evocation.

But all characters are not created equal.When i creat a 9 refresh mortal character he has way more influence over the story then the wizard. The wizard has not even control over himself because he constantly has to take compels why the other players declares stuff left and right.

A wizard is STRONG and SCARY in combat (if build this way) but not every situation dissolves to combat.
Fate is story first and balance second,