(This is an excerpt from the comprehensive post on Napoli, which details all of the Gate Loot, Job Loot, and Boss Fights from Bronze through Ruby).
*Defeating Don Enzo at Ruby Level*
At Ruby Level, Enzo starts with an intimidating 36,000 Health. You're best served asking for help - or better yet, coordinating with a Mafia member or two when you're about to start this fight, so they can help out. You have 6 hours to defeat the boss, and this one will take a lot of recharging your Health.
How quickly you complete this depends on (i) how many Mafia members you can enlist to help you as you fight; (ii) your Health, Attack, and Defense stats; and (iii) how much Stamina you start the fight with (make sure it is more than 500, to be safe - you use it each time you attack, and each time you return to the fight).
Unless you're Superman, you'll see this screen many times during your fight:
I saw it probably 25 times. No worries. Also - don't do jobs to reload on consumables when you lose. I had a Mafia member help (thank you DF!) and simply followed this pattern to victory, in about 30 minutes:
Step 1. - Get rid of the Henchmen with your Grenades. This took me more than one "life" to do, but they have to go first.
Step 2. - Each time you are crushed, go to New York/Cuba/Moscow to heal. Since you can't heal instantly, do a couple of menial tasks (rob a property, etc.) and heal as soon as you can. DON'T LEAVE MAFIA WARS. Then return to Italy and go right back to the fight with Don Enzo.
Step 3. - Use the best available attack sequences depending on your remaining consumables. With Don Enzo at this level, I found that Health Kits were useless. My attacks were doing about 560 damage apiece, while his did 640. I only have 800 Health, and after the first hit from Don Enzo, I'd have 160 Health and the kit would bring me to maybe 350... still not enough to survive another hit. If you have better Health or Defense, your Health Kits may buy you another round with the Don in one fight.
I used these sequences, in order, going back to Step 2 each time I was beaten:
 STUN GUNS: I would get in 4 hits in one "life" by using a Stun Gun/Attack/Attack sequence first, then getting hit by the Boss, then using the same Stun Gun/Attack/Attack sequence. That would take him down about 2,100 Health each time, though I could only do that 5 times with my 10 Stun Guns.
 SHIVS: I could get in 2 rounds of Shiv + Attack each "life" once I was out of Stun Guns. That would take him down about 1,800 Health each time, but again, I could only do THIS sequence in 5 "rounds" with the Don.
 GRENADES: Same sequence as Shivs, but less damage - and I only had 8 Grenades left after dispatching with the Henchmen - so the Grenade + Attack sequence, twice in each round, did maybe 1,600 damage to the Boss' Health in the 4 times I could do it.
 ATTACK! Once the consumables were gone, I just hit Attack, got hit by the Don, hit Attack again, and went back to Step 2. This did about 1,100 Health damage each time and slowly depleted the Boss until....
Enzo's Monocle has 104A/155D at Ruby level... and will replaceBidwell's Suit (from Las Vegas Boss Fight) as your top defensive Armor piece.