“彻底打败”可以用以下几种英语表达，具体取决于语境和想要强调的侧重点：
1、 completely defeat：
强调“完全地、彻底地”击败对方，不留任何余地。
例句：The army was completely defeated in the battle.（军队在战斗中被彻底打败了。）
2、 thoroughly defeat：
“thoroughly”表示“彻底地、完全地”，与“defeat”结合，强调打败的彻底性。
例句：They were thoroughly defeated in the game.（他们在比赛中被彻底打败了。）
3、 utterly rout：
“utterly”表示“完全地、绝对地”，“rout”意为“彻底击败、击溃”，常用于描述军事或竞技上的惨败。
例句：The opposing team was utterly routed.（对方球队被彻底击败了。）
4、 wipe out completely：
“wipe out”意为“消灭、彻底摧毁”，加上“completely”后，更强调打败的彻底性和毁灭性。
例句：The enemy forces were wiped out completely.（敌军被彻底消灭了。）
5、 annihilate completely：
“annihilate”意为“彻底消灭、歼灭”，是一个较为强烈的词汇，常用于描述极端的打败情况。
例句：The enemy was annihilated completely.（敌人被彻底歼灭了。）