There are blows in life, so powerful…I don't know! (1)
When the speaker talks about hard times in life he uses the word "blows"—repeatedly. We can't ignore the fact that this word itself is violent—it means hostile striking, hitting with a closed fist. It's meant to cause pain, and is a violent way for life to treat poor little humans.
[…] They open dark trenchesin the fiercest face and in the strongest back. (4-5)
The blows come back here, and this time they are slicing into skin. The way they open up dark trenches makes us think of a plow digging up the earth, or a whip cutting into flesh and leaving deep marks. Even the strongest, fiercest ones can't escape life's blows.
Perhaps they are the colts of barbaric Attilas; (6)
And in case you thought the blows were a force of nature, here we get them tied up to a fierce warrior. The link between the "barbaric Atillas" and the blows is that both of them are wildly violent and terrifying.