Elise with 400 AP vs an 2600 HP opponent at lv18. With your penetrations+enemy MR, let's assume they have an effective 80 MR. That reduces her damage by 45%.
750+595=1345 damage by Q+W. Reduce that by 45% and you will deal exactly 740. Her Q in spider form will now deal 378 damage + 80 damage for each spiderling, which is a total of 778 unmitigated, and 427 mitigated damage.
That's a total of 1167 pure damage to an enemy if you had 0 penetration, and they 80 MR.
And then, you use your W to do a few attacks on the enemy.
So let's give you three seconds to AA the enemy with your spiders.
If you didn't buy any AS items, you will have 1,7 AS for those 3 seconds, and your spiderlings will have 0,93. In 3 seconds you will do 5 auto attacks, while your spiderlings will do 3 approximately.
You will deal 101 damage per attack with your base damage and an additional 160 magic damage with your ult, while your spiderlings deal 80 damage per attack, each.
In total you will deal 505+800 with your AAs in those 3 seconds, while your spiderlings deal 1200, if allowed to attack 3 times.
From her R+W in spider form alone she deals a total of 2500 unmitigated damage. But mitigated, it turns to 1100 mitigated magic damage + 500 unmitigated physical damage, but that one we won't count, for now.
Her other abilities all together deal 1167 pure magic damage to an enemy with 2600 HP and 80 MR.
Her R+W in spider form can deal 1100 pure damage to any enemy regardless of HP and with 80 MR.
Her ult's on-hit scaling is OP for both herself and the spiderlings. The other abilities are fine.