Well technically Flak is a seriously powerful weapon... 200 damage per second. I don't know how efficently that is transferred to a warship sized target however. Based purely on that statistic however, it beats out the SGreen by nearly 140 damage a second, and is about as effective as an SVas.
Be that as it may, I'm reasonably sure the Fenris would be victorious against four or so Tritons. And the AAA from the Fenris might just keep the escort off of it long enough to complete its task.
If we were going to be really honest about it, a Myrmidon can p0wn every non fighter warship short of a Sathanas in the game