Translation: "To Say The Least"
Word for word translations are not always available or desirable. 至少/起码可以说 works, but is foreign in scope. What you want is a phrase that catches the essence of it, like 说得好听, or 毫不夸张地说, or perhaps 坦率地说. 退一步说 is fine, too.

Shorter and more to the point equivalents are 说少 and 少言之.

打折扣地说 and 轻描淡写 both mean understatement, which is another way of phrasing it.