This is a tough fight, even on the easier difficulties. Marc Antony has a lot of HP and several unblockable attacks. There is no substitute for learning his attack patterns and using basically every resource at your disposal.
I'd highly recommend buying the counter skill before this fight (level 1 unholy) - although he has a lot of unblockable attacks, some of his attacks are blockable and easily countered. Also, at least one health upgrade will make your life easier as well.
He has a number of different attacks which he mixes up almost at random, so this guide will cover each of those. The basic trick is to evade his attacks using the RS, or counter if possible, and hit him straight afterwards before he gets into his next attack.
1. Four sword slashes. The first three can be countered or blocked, the last one (burning sword) must be evaded.
2. Twirling his sword around. Actually not an attack at all, he will showboat and do this occasionally - this is the best time to strike him as he is totally vulnerable.
3. Glowing shield plant. This inflicts only a small amount of damage and is blockable (I think). However, once his shield is planted you can't hit him (please let me know if you have discovered any ways to break his guard). From here he will pause for a few seconds and then launch into either a jumping flaming sword attack (unblockable, must be evaded) if you're close, or a vicious ranged attack (can be blocked or jumped) if you're distant. The trick here is to stay close, evade his sword attack, and then rush in on him.
4. Sword spikes. Here he plants his shield and a wave of swords spike up from the ground. IIRC this is blockable. However I found it safer to evade as this one can do a bit of damage if you're not careful.
5. Health recovery. When he starts to glow green (this happens at roughly 75%, 50% and 25% health), Semiramis is recharging his health - you need to go and slash her hand to stop this. Doing so will regenerate your health also.
6. Lust storm. Occasionally Semiramis will send out a purple storm into the fight which will follow you around. Basically you need to dodge this and draw it to Antony. It will 'stick' to him - you then need to just run away for a bit. I somehow managed to counter this at one point but I'm not sure exactly how I did it.
7. Sometimes Semiramis will try and crush you with her fist - this only happens towards the top/middle of the arena so best to stay near the bottom if possible.
8. Ranged attack - you can try and chip his health using B attacks from long range, but he tends to respond with the ranged attack mentioned at (3) above which does quite a lot of damage and can be hard to see coming, so I'd advise against this approach.
UPDATE: Several posters have commented that if you have the cross reasonably well-powered up early on, he is very easy to beat. This could well be true - I haven't tried this approach.
9. Make sure you use your redemption and magic, particularly later in the fight as he will use the easily-avoided attack (1) less and less and your opportunities to hit him will be fewer and fewer.
He may take a bit of practice to beat - I've tried to describe his attacks above but he does mix it up a lot so there is no substitute for actually learning how to telegraph his attacks.