Demeter's cast does 8-11 base damage every 0.2 seconds for 5 seconds, 5 ticks per second for 5 seconds is 25 ticks, 8x25 = 200, 11x25 = 275. 200 to 275 damage per cast. You will never get that much damage out of it because it tracks so slow.
The Artemis duo adds 10% damage and makes it actually track, so now you're actually getting that 220-300 maximum damage on each cast.
Lightning phalanx does 85-136 base damage in a moderate AoE up to 5 times, theoretical maximum of 425-680 damage if it doesn't catch anything in the AoE. If you get more than one enemy in the AoE, your damage starts increasing exponentially. Athena's cast embeds, so there's a longer cycle time if you don't kill what it embeds into or it bounces somewhere far away, but it can also have a faster cycle time if you kill the embedded enemy before 5 seconds are up.