my gut instinct is this because I think tricks up one's sleeve would have a direct equivalent in Spanish

Well-armed, metaphorically speaking, means the person is well-prepared for any eventuality or difficulty.



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Note added at 9 hrs (2014-09-17 06:05:48 GMT)
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in view of your addition of context 'atrabancada' I would rephrase like this using two idiomatic forms together which render the idea of 'aggressive, dangerous, reckless' and both with military origins.

A force to be reckoned with, she came well-armed.

The tone of the resulting sentance in general means...you had better watch her because she knew how to fight her own battles.... in fact, now I think about it you could actually also say:

She knew how to fight her own battles and she came well-armed!

ie, somebody who is more than capable of defending herself - a force to be reckoned with

Also: someone who gives as good as she gets

someone you don't mess around with

feisty

Here (with atrabancada): someone with a short fuse and feisty with it

Other synonyms that might work: gritty. gutsy, spunky, sparky…

OR someone with a short fuse and who can give as good as she gets…

OR indeed, any combination of any of the above

As others have said, we really need to see this phrase in context to decide the best way to say it in English. But as for the meaning, it's along these lines.

It literally means someone with a lot of cartridges on his/her belt, like a bandit or a cowboy in the Wild West. But it's used figuratively, and the idea it conjures up is that of a person who will tackle situations head on, fearlessly, and will not be deterred; someone who will lay down the law.

Here are a couple of examples to give the flavour of it. This first one is describing the classic "mujer de armas tomar":

"Eloisa, la madre, con el delantal sucio, el moño recio, las cananas en la cintura y la escopeta de caza al hombro"
http://labasquebondissante.blogspot.com.es/2010/06/mi-top-10...

This next one is about corruption in Spain and the fact that nobody is doing anything effective about it:

"Solo decir que como no venga una persona con cananas en la cintura esto no hay Dios que lo cambie. Se tapan unos a otros y se pasan de juicios en juicios mientras los delitos prescriben."
http://www.taringa.net/posts/economia-negocios/16770085/Econ...

The mother is "a very tough cookie". What Spain needs is someone who will sort things out and "won't stand for any nonsense". These are the expression that come to my mind.

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Note added at 20 hrs (2014-09-17 17:00:14 GMT)
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Yet another possibility, depending as always on the context: "someone who means business".