IMO, the Forsaken's strength range from 80 - 100 (Moghedien is at 70). Mesaana, Asmodean and others are on the lower side, and Ishamael, Lanfear and Demandred at the top.

Aes Sedai strength ranges from 5 to 25, with most Sisters sitting between 15 and 25 - which keeps with Siuan's statement on strength. A strong sister is at about 25, average at 20, weak at 15, and some exceptionally weak women are below this.

Depending on which arena you play in, this is all acceptable. Siuan/Moiraine is about 20 - 25% stronger than the average Aes Sedai, and the strongest Forsaken are about 20 - 25% stronger than the average Forsaken.

Elayne (50) + Merilille (20) + Renaille (20) = 90
Egwene (50) + Romanda (25) + Lelaine (25) = 100
Egwene (50) + Aviendha (50) = 100

Of course, both circles lose a little strength due to linking, but gain the advantage of a singular precision flow. Egwene has seen 100 strength in action, so she's worried she couldn't stand up to the Forsaken even with two strong women linked to her. Elayne merely thinks she can tear down half the palace, which Taim could do too - it's a pretty even match.