skillful means

Upaya and Achieving Buddhahood

|...the meaning of "the union of method and wisdom"?

a guess: 'upaya', 'skillful means'

|...one can realize the inherent emptiness of reality.

if one is empty, there can be no realization, for there is no one to
 realize this emptiness and no 'reality' (as it is also empty of own being)
 to be empty.  'reality' is not something about which realizations may
 occur in any case, since 'reality' is an intellectual abstraction 

  ('empty' is the rough translation for 'sunyata' I presume; this
    term does not mean devoid of substance but instead lacking in
    individual and separate existence (own being))

|...I do not underatand that one can achieve Buddhahood 

that would depend on the phrase 'achieve Buddhahood' 

on the surface this seems an innocent pairing, and yet 'achieve' implies 
 an arrival, an endpoint.  is Buddhahood a static endpoint?  is it 
 something like 'complete awareness'?  is there such a thing?  how about 
 'perfect awakening'?  do words really do the experience justice?

|through engaging method or means. 

there is a story of means as dualism to pluck dualism out of oneself.
 this appears to instruct the method along taoist lines:

	when goal-tool affords success, discard goal-tool

in tao stories often it is a net, catching fish
 there are variations throughout the world
 and yet, where does the method stop, the goal begin?
 the subject is overly complex for persistent rudimentary
 analysis (such as my own).  I imagine all this is covered
 by the masters somewhere in a Buddhist cannon (BOOM!)

|...the means of achieving Buddhahood and the goal, Buddhahood 
|itself can be achieved at the same time?

the means may be achieved, but this achievement must eventually
 be abandoned, accepted and released in order that the goal may 
 as well be abandoned, accepted and released 

|[what does 'upaya' mean?]

as I understand it (just reviewing _The Shambhala Dictionary of 
 Buddhism and Zen_), upaya is a boon granted unto us by the
 bodhisattvas - the demi-buddhas who have returned from the brink
 of nirvana in order to assist all of us in experiencing It too.

these Benefactors of wisdom and charitable karmic influence weave
 their way through the cosmos enhancing the experience of a horde 
 of people like you and me who are reborn into countless lifetimes 
 and weaving through the Samsaric Merry-Go-Round in a bittersweet 
 yet beauteous buddhanatural dance

the way Mr. Gotama has us cataloging things 'upaya' is a justification
 schema for instituting religious changes and a personal argument for
 doing anything you want in association with your path of awakening
 
there are 8-fold pathways growing out of your shorts.  who said you
 weren't a buddha?  the cosmos burgeoning with awakening medicines,
 how can we help but begin to see the value of individual methods?
  
'upaya' are the various methods by which awakening may take place