Tail Spike: While underground, Serkets shoot their tail up from below, along with their whole body, to impale any player above them; this exposes their weak point for a few seconds before they dig underground again.
Claw Burst: While underground, Serkets would pop out from the ground claws first, along with only their head; they stop for a few seconds, but it only exposes their hard exoskeleton.
Deadly Embrace: Serkets click their claws then chase any player from afar to slash him/her with their claws in a manner similar to an embrace.
Tail Impale: Serkets would stab their tails on the ground first to the left of their heads, right, then center getting their tails stuck while leaving their deadly toxin on the ground, which remains for quite a while.
Body Swing: Similar to Tail Impale, Serkets would then instead rotate 360° rather than stab in front of their heads.
Using damaging Ultimate Spells like Bahamut and Grendel or status effect inducing Ultimate Spells like Agni would instantly bring the Serket out from the ground.
Attacking it from the back when the Serket's tail is stuck on the ground or when it does Tail Spike is the only way to attack its underbelly because the legs block it. Being in the right position, depending on the weapon, would ensure that every attack by the player will hit its weak point even when the Serket is toppled at its sides when it is stunned.
Flame cards also work well with either version of Serket as the burn damage continues for ~2 seconds even after it digs itself back into the ground. Thunder cards are useful to paralyze or stun the Serket for a few seconds, enough to cause them to topple to their side and give the player a ~5 second window to attack the underbelly freely.