I usually see this phrase in Shonen manga from an antagonist, referring to a "persistent" character.
I remember a quote that goes like this:
ほう 大したもんだ。
It was from a character called "Ohm" from One Piece, when found that Roronoa Zoro was still alive.
大した is used to express admiration for something one finds remarkable or extraordinary. 大したもんだ or 大したものだ is kind of like saying “You are something” but it sounds condescending. It may also be used sarcastically.
This is a set phrase that literally means "That(it) is a big deal." But the connotations and actual meanings depend on the context.
A breakdown of the phrase goes:
大した
considerable; great; important; significant; a big deal
もん＝もの
thing
だ
copula
You can use this expression to praise the someone else by marveling at their achievement.
Similar expressions
大したものですね/大したものだね/すごいですね/すごいね/さすがですね/さすがだね
There is another expression 大したことではない/大したことない meaning "not a big deal".
