Here is damage information about the minibosses (well, one's not a miniboss...):
sea sub (Octopus)
/64
XB 2&4:4:5:6
ST 1
FW 0
SI 4:8
ES 6
RS 8:6
HT 2:8
BC 6:10
CS 8
sea serpent (Octopus)
/72
XB 1&2:1:2:1
ST 1:2
FW 0
SI 1:2
ES 1:2
RS 1:2
HT 1:2
BC 3:4
CS 1
thunder slimer (Mandrill)
/48
XB 1&2:2:4:4
ST 1:4
FW 1
SI 2:8
ES 2:6
RS 4
HT 3
BC 2:6
CS 2
grapple bot (Chameleon)
/64
XB 1&2:1:2:1
ST 1:2
FW 1:2
SI 1:2
ES 1:2
RS 1:2
HT 1:2
BC 3:4
CS 1
exploding blockades (Eagle)
/32
XB 1&2:2:4:4
ST 1:4
FW 1
SI 2:8
ES 2:6
RS 4
HT 3
BC 2:6
CS 2
The number after / is the HP of the miniboss.
XB: The : separates charge. Number after & is dash-shot.
Others: Ordered starting at Storm Tornado and going in order of boss weakness. The : separates charge. If no second number given, charged form (if any) does same damage as uncharged.
Storm Tornado is still better against the sea serpent than Boomerang Cutter.