User blog comment:DeathGodPrayer/Talk About What You Want!/@comment-213.106.131.123-20131031152845/@comment-213.106.131.123-20131102104725

To answer the question above the one above

Ways Issei could acquire a Sub-Species "Boosted Gear" Ganutlet:

1. His Sacred gear fuses with Euclid's

2. Someone he cares about gets killed (or so he thinks) infront of him, I don't think he can go "Juggernaut Drive" due to the fact their are know memories residing in "Boosted Gear" anymore, so he awakens a new power

3. Oppai/Ecchi related power up (most likely one to happen)

4. Intense physical training in his base form

5. Intense mental training/Meditation (most unlikely one to happen)

There are probably more ways, these were the ones a could think of