That's interesting. I have a lvl 22 Basilisk that I've mostly leveled by merging with other basilisks. He has 278 ATK and 270 SPD, but only 720 HP. He's a core member of my rotation, but I never put him in the leadership slot because I figure that people are looking for a support skill and higher HP, even though he's kinda awesome.
Yeah, that thing is a beast, though a bit unfortunate it doesnt grow better HP.