Here comes the fun. With a base mana at level 18 of 1075, plus the 1000 of Muramana, plus the 500 of Iceborn Gauntlet (or 250 of the Trinity Force), we should have between 2325 to 2575.
The thing is, if with Muramana toggled on a 3% of Mana is consumed to dish out a 6% of it in AD, we should get that we consume about 70 mana to get about 140 damage at least (with Trinity), or 78 mana and 155 AD at most (with gauntlet).
The fun part is, with an Essence Reaver, every attack crit or spell crit (Q) recovers 3% of our maximum mana (always more than the 3% of CURRENT mana consumed by Muramana toggle). That is, your mana doesn't deplete as long as you have 100% crit.
Having capped our CDR at 40% (we bought 50%), we get a 2,7 sec cooldown on Q.
On attack speed, with the full build he has +75% increase of his base attack speed, which at level 18 would have grown by +47,5% as well, which amounts to +122,5% of 0,625 = 1,39 AS.
That, along with his passive, which stacks another 50% bonus AS, would add up to 1,70 AS. Pretty high.
With 97 base AD at level 18; we've also got 65 from ER, 65 from IE, 20 from Maramune, plus it's passive, 2% of 2575 (=51,5), which is 298,5 AD in total.

THE AUTOS: Taking into account this can (and will) crit, with the IE boost (250%), it amounts to about 750 Damage per auto. Plus the extra 150 from the Muramana toggle, ¿total of 900?

THE Q: Since it's a 1.1 AD scaling, this is a base damage of 115 plus around 328 (=298,5 AD x 1.1), total of 443. SPELLBLADE: don't forget that nice Sheen in the Gauntlet (125% base AD) or even better, in Trinity (200% base AD). Let's do the math with Gauntlet, since we've done everything else with its AS, Mana, etc. That would be a plus of about 121 Damage to at least every Q, plus the extra 150 from the Muramana toggle, that also applies... that's 714 Damage from a Q.

THE W: another 250.

THE E: Not to be underestimated, it has a base damage at level 5 of 275 plus 50% of Bonus AD, so about an extra 100 damage to a total of 375. Plus the extra 150 from the Muramana toggle :'). Yeah. Another 525.

CONCLUSION: There you have it. As soon as he gets on your face he can deal more than 2400 damage. Sure, you can mitigate it and survive, but in the next 0,7 seconds he's gonna have fired another auto of 900. And it's only gonna get faster. I mean... Gotta love the numbers. Who needs boots, right?