Synonyms for Flatten:

n.

obliterate, eat away, ruin, shatter, trash, ravage, mutilate, vandalize. flat, beat, overpower, straighten, win, overcome, defeat, knock out. curl, work, bend, shape, ground, deform, form, knock-down, rise, bring down, twist, reshape, open up, prostrate, deck, floor, down. press, rub, nip, squeeze, scrunch. knock, punch someone's lights out, lay out, take down. flatten (noun)
drop, flatten out.
level (noun)
floor, ground, straighten, even out, roll out, prostrate, depress, knock-down, level out, abrade, beat down, steamroller, squash, level off, crush, plaster down, wear down, spread-out, smooth out, debase.

v.

change (verb)
drop.
destroy (verb)
demolish, bulldoze, rip to shreds, eradicate, wreck, atomize, dismantle, raze, smash, destroy, devastate, vaporize, annihilate, abolish, lay waste, pulverize, level, disintegrate, extirpate, decimate, tear down, desolate, mow down, exterminate.
flatten (verb)
compress, squish, deflate, grade.
level out (verb)
depress, straighten, squash, abrade, compress, mow down, grade, smooth, ground, debase, deflate, floor, plane, beat down, raze, prostrate, even out, crush, smash.
smooth (verb)
gloss, silken, slick, sand, polish, glaze, burnish, plane, smooth, grind, even.

Other synonyms:

depress. nip. floor, prostrate, deck, straighten, scrunch. knock-down, knock out. rub, take down. crush. lay out, squeeze. flat. knock, press. destroy
pulverize.
push down
depress.
smooth
sand.
unroll
roll out.
Other relevant words:
even out, lay out, steamroller, crush, down, ravage, bend, floor, depress, flat, ruin, drop, mutilate, level out, knock, knock-down, overpower, squash, level off, abrade, smooth out, debase, press, beat down, prostrate, flatten out, deck, shatter, take down, straighten, roll out, nip, bring down, beat, obliterate, defeat, ground.

Usage examples for flatten

  1. Tom's object, then, was to flatten the trajectory, by lowering the muzzle of the gun, in order to attain greater distance. – Tom Swift and his Giant Cannon or, The Longest Shots on Record by Victor Appleton
  2. Her look once focused, she began, little by little, to straighten, to stand more firmly on her feet; she even reached to flatten the starched collar, which had upreared behind her slender throat. – Apron-Strings by Eleanor Gates
  3. Flatten out the bottom fold as you did for the frog's back, then bend the ends and sides as in Fig. – Little Folks' Handy Book by Lina Beard Adelia B. Beard