'Attack' means how quickly a note reaches its full volume. For instance, the background synth in the intro of Circus-P's 1/4 will have a high attack, since it doesn't reach it's full volume immediately as the note is played.
In UTAU, that could mean one of two things, since 'attack' isn't a feature. This could mean adjusting the oto's as you've mentioned, by cutting out the silence at the beginning of the recording and making sure not too much vowel makes it into the 'consonant' part of the oto.
Otherwise, if that's all fine and dandy, look at the consonant velocity. This allows you to adjust the speed of the part of the recording you've highlighted as the consonant. This is probably what they're referring to. The default setting for this is 100. Guren no Yumiya is quite a fast song, so I'd try adjusting it in increments of 25 until the lyrics sound clear.
Hope this is what you're looking for!