So, if your thesis is:
In his poem "Dulce et Decorum Est", Wilfred Owen portrays the atrocities of war through his use of connotative diction, structure, and auditory imagery.
Then your topic sentences will sound something like this:
TS for body 1:
Through the use of connotative diction, Owen conveys the idea that war is truly terrible.
TS for body 2:
Owen's use of structure in "Dulce et Decorum Est" likewise helps to display the horrors of World War One to readers.
TS for body 3:
Auditory imagery throughout the poem also serves to highlight the atrocities of war.