The end "boss" of YTM Chapter 5, the "Taurus" is pretty much the toughest enemy I've encountered so far in the game.
 
There is only one way I've found to beat it and that was to get in its face where his weapons could not reach it. Problem is when you are using mostly battleships most of your weapons will not fire at that range. Now it would be possible to simply park your fleet in front of it and fire whatever weapon that can hit it at that range for a long time but the Taurus' special abbility transforms this tactic in a waste of time. The Taurus can repair itself for 4000 HP in one go.
Trying to take it out from afar is pretty much suicide too because even my armour ship that had about 6000HP and 130 defence was blown to pieces in one barrage hit from the Taurus. 
 
So what to do against the Taurus? Sell your whole battleship fleet how painful that might be. 
 
I sold 3 of my battleships and bought 2 carriers back for it and filled it and my main ship with Intrigo fighters, a total of 155 of them. And filled my 3 ships with Hangars to release as much as them as possible at every assault and a fast engine to close up that gap between you and the Taurus as fast as possible. The 3 ships I ended up with was my well loved Erebus, a Eberlin and a cheaper Daphne. 
 
It still took me a good 10 min even with the about 190 damage per sec from the planes (eventually that number slimmed down to 120 as they were shot down) and pounding the 3 short range cannons that could hit it. 
 
At least you get 30000 Credits and about 500xp from that battle but you'll be stuck with a lot of fighters afterwards.