I got this on the boat section of the Jungle Rip level. Just abuse the save system and reload if you get hit. You can save as many times as you like.
Once you beat the first section of the sub boss it's very easy to take down, also during the battle you are able to use rockets (LB) even though I found I couldn't use them for the rest of the section beforehand. This gives you a spread shot which makes it easier to hit the sub.