Thanks for the tip -- partner and I have gotten to fight with Annuvin, but can't even seem to get ANY kind of ending aside from us both dying.....we're just not doing sufficient damage to Annuvin, have tried a number of approaches...both characters have drunk Sleg (E once, C the rest of the time). Does anyone think this failure at the boss fight is because both characters have drunk the Sleg, or is there something we've missed along the way? Just wondering.....we're going to try a re-do without drinking the Sleg and see what happens.
It sounds more like you haven't found the 'trick' to beating the final boss. It took my co-op partner and I about 4-5 retries to finally notice what was going on in that fight and what was needed to win. Simply put, brute force isn't going to cut it. Observe what is happening to Annuvin and the surrounding level's graphics — those will give you the hints you need to beat it. Long story short, you need to have good teamwork and take turns protecting one another to succeed.
If that still doesn't help, try playing it in single-play as E'lara and see what Caddoc tries doing after you've gotten the boss' health below 75%.