Playing as Great Cthulhu
Your Spellbooks and abilities, even unused, exert a baleful influence on your enemy’s activities. Submerge, Dreams, and Y’ha Nthlei all cast a pall over the map, hindering your opponents.
You get Spellbooks quickly in the early game. Get Cthulhu out on Action Phase two or three, even if he is vulnerable to an enemy attack. After all, you can re-Awaken him cheaply and gain an Elder Sign. It is far easier to get your two “Kill/Devour” Spellbooks once Cthulhu has taken the field.
In the late game, use Submerge to strike critical enemy Areas. With Regeneration or Absorb, plus Cthulhu’s Immortal ability, you can hurl your forces into Battle with impunity. Cthulhu only rolls six dice, but an Absorbent Shoggoth gives you heft; Cthulhu’s Devour ability is better than a free Kill.
Cthulhu’s Faction is focused on destruction. Of all Factions, your Units are the most effective in a fight. However, your access to Elder Signs is limited, so you will need to use your combat strength to ruin your enemy’s bases and keep them from taking the lead.
Absorb
Turns Shoggoths into major Combat dice. You only need one Shoggoth per attack, as long as he has “food” available. Remember that Cultists and Deep Ones are also useful for soaking up hits, so you may not want to Absorb them all.
Devolve
Keep a Deep One in your Pool to render you immune to Capture. Just Devolve a threatened Cultist.
Dreams
This means that your enemies dare not leave a Cultist alone on a Gate. You may not use Dreams often against experienced players, but it will affect their behaviors throughout the game.
Regeneration
With this, Starspawn inure your army to harm, making them the flip-side to Absorb’s massive damage output.
Cthulhu accompanied by two Starspawn is well-armored against even enemy Great Old Ones.
Submerge
A critical Spellbook in the late game. Once Cthulhu’s army Submerges, not only is he safe from harm, but other players react in desperation. If you attack an ocean Area, you can Submerge again after the strike.
Y’ha Nthlei
Like Dreams, this inhibits your enemies. Your foes are less likely to conquer your ocean Gates, and you don’t care as much if they do.
Thwarting Great Cthulhu
Cthulhu has strong Units, but not many of them. Each loss is a setback for him. When he hurls his might against an Area, he will win. You can mitigate this by not putting all your eggs in one basket. Killing Cthulhu himself isn’t particularly effective (unless you are Crawling Chaos, since Harbinger then gives you two Power or Elder Signs), because soon he’ll be back at R’lyeh, ready to Submerge and make your life hell once again.