I have played a few games of arkham now and have most everything down. There is however one thing that eludes me, why would you summon the ancient one early? There are several cards that do this including spells and tomes, I can mostly think of them as an end the game because it takes too long but, Kate winthrop's personal story if successfully completed all but guarantees they awaken, am in just missing something.

( i originally thought they would have less doom tokens, however according to the rules he fills the doom track when awakened)

If your Investigators are all (or nearly all) Blessed, Joe Diamond has the Fight Skill and a pile of Clue Tokens, Lily Chen is part of the group or the Investigator who can remove a Doom Token during Upkeep (and twice if she's passed her Personal Story), you may want to call the GOO forward a bit earlier than planned.

Maybe you've got the AO's –X modifier in a spot you'd like it (Hastur and terror level; Yibb-Tstll and clues on the board) and, anticipating battle, you'd like to leap on your advantage.

Also, call the ancient one spell can severely weaken the Old One.

Thank you all, clearly I need to rethink fighting the ancient one as a strategy as we usually focus on sealing gates.

Shudde M'ell would be good to summon early and kill, since he can end the game without even showing up. He's also relatively soft and squishy for an AO.

Sealing the gates is still the best way to win, assuming you have at least 4 investigators. But… if you draw (or choose) powerful fighters, a couple of decent weapons and a way to summon the ancient one, it would be a good tactic. Take time to bless your team, though. A few good allies can also come in handy. There are also ancient ones that you don't want to summon, most of them from expansions, such as Quachil Uttaus and Cthugha.