Copying/pasting from elsewhere in this thread because I keep seeing this inane and absolutely, demonstrably false bullshit spewed all over. You can't maths and you clearly VASTLY underestimate how much fucking damage there is in this game, and how little defense matters, even when not properly penetrated.
"Ryze, Azir, and Cass can easily do upwards of 1k DPS, especially single target and at full build; in fact, even Cass who does significant AoE and who CANNOT itemize over half of the flat pen she wants still does nearly 2k DPS.
First, let's look at what Mundos actually build:
In Plat+ here is his most common build:
/
Just to strongman your argument, we'll go with the Randuin's to increase his mEHP.
This gives a level 18 Mundo:
+425HP, +450+55MR, +800HP, +40MR, +400HP for a total of 95MR, and 2075 bonus HP.
Mundo's base stats are: 2095HP, 53MR
Adding those together, we have: 4170 HP and 148 MR.
~
Let's use Cass as an example, as she will have the lowest numbers of the champions you have mentioned.
Note, she can not get 18 flat pen from boots, and does build Morello and Void Staff both extremely commonly, in essentially every game that goes long enough in Plat+ according to League of Graph.
Thus, against her, Mundo will have 73.8MR (for the other champions who buy sorc shoes, this would be further taken down to 55.8MR, or only 37% damage reduction) 73.8 MR provides 44% damage reduction
Thus, to kill Mundo by herself, Cass will need to deal ~7,425 pre-mitigation magical damage.
How long will it take her to do so? Less than five seconds, actually, but let's break it down.
Well, we need to look at her build first, clearly. Her most common build is:

Right off the bat the maths gets muddled as we have multiple %increased moddifers (three to be exact, which happen to all scale exponentially off each other) plus %HP damage. Luckily, we know the HP of our target, so we can actually calculate this. furthermore, the Rylais guarantees that Liandry's will always tick for its increased damage rate of 2.5% of Mundo's max HP (104.25 damage per second, in fact, this burn alone would kill Mundo in roughly 40 seconds; BUT WAIT, for every second in combat, Cass does 2% increased damage, so even the first tick is actually 106.335, the second tick is 108.42, the third tick is 110.5, the fourth tick is 112.6, and the fifth and later ticks are 114.675 damage Thus Liandry's alone deals 552.53 damage over the first 5 seconds, and 114.675 a second for every second afterwards; Liandry's alone kills Mundo in 36.5 seconds.)
With that build her mana will be 2353.5 at level 18, which gives her an additional 70.6 AP. Her items give her: 50 + 90 + 75 + 70 + 70 + 120 + that 70.6 AP from Seraph's passive = 545.605 AP, before Rabbadon's Deathcap, which increases that by a further 40% to 763.847 AP
~
Her Q deals 215 (+ 90% AP) over three seconds, thus with 763.847 AP, she will do 902.4623 damage over three seconds, or 300.82076667 damage a second
Her W deals 40 (+ 15% AP) damage a second, so with 763.847 AP, she will do 154.57705 damage a second
Her E deals 120 + .1 AP plus 90 + .6 AP if poisoned (hint, Mundo always will be) every three quarters of a second, for 744.6929 raw damage, or 992.9238667 damage a second
For a total of 1448.32168337 damage per second (not counting Liandry's amp, nor its burn) which alone seems ludicrously high, but wait, it gets better.
Her ult deals 350 (+ 50% AP), and she'll only get it off once, so we'll include it as burst damage 5 seconds in when she's got her Liandry's fully powered up. In which case, it will deal 731.9235 base damage increased to 805.11585 with Liandry's.
~
Finally, to put this all together:
(don't worry I'll add Liandry's ramping damage amp to the DPS calculations)
Cass deals:
552.53 damage over the first 5 seconds from Linadry's burn
805.11585 damage from her ult
These alone account for 1357.64585 of the 7425 pre-mitigation magical damage needed to kill Mundo.
The first second of damage she will deal another 1477.2881170374 damage.
the second second she will deal 1506.2545507048 damage
The third second she will deal 1535.2209843722 damage
the fourth second she will deal 1564.1874180396 damage
The fifth second she will deal another 1593.153851707 damage
Unfortunately, she quite comfortably overkills him, having done over 9,000 damage in 5 seconds (9033.750771861 damage to be exact).
Remember that this Mundo, at level 18 and with a full build only has 7,425 mEHP. In fact, Cassiopeia can 100-0 this Mundo in under 4 seconds if she ults a lil' earlier than ideal.
I won't bore you with even more math, but some of the champions mentioned can kill him 100-0 by themselves in under 3.5 seconds.