Before I solve this word-riddle, let me give you a few hints on how to attack such a problem: first, every syllable needs at least one vowel or the letter “Y”; next, certain letters never or rarely touch others in a certain order in English: bt, nl, etc. Third, English has some seldom-used lettrs: z, j, x, q, for example. Last, it only takes twenty seconds or so to go through the alphabet in your mind, letter by letter. If you write down these letters one by one and cross out the ones that simply don’t occur in English, you will be left with a manageable bunch.
Now let’s take your riddle: _r_n_ed
The first missing letter could be many, so leave that for later.
The middle two could also be many, so leave it.
The last missing letter cannot be a, b,f,g,h,j,l,q,r,s,u,v,w,x, so must be c,d,e,i,k,m,n,o,p,t,y,z.
"Trounced" -- does that fit the clue? No, because it is 8 letters and "trounced" means to be beaten severely, not tensed with embarrassment. Try c-r-a-n-k-e-d up.