"Unless we can trick it into hitting itself?  Or find a weak spot in the armour, like an eye or a joint?  I dare say we don't have anything stronger, no.  It seems like the safest place may be near it, though, where it can't point that thing at us."

Deciding his rapier won't help, Khond opts to fish for his tinderbox and see if he can play along with Rick's plan.
(No clue if this will count as "abundant, exposed fuel," but might as well try.)

As the realization hits that their weapons at least seem unable to pierce the "frog"'s armor, a variety of plans gets set in motion. Breldo keeps to the back, standing ready in hopes that if the thing tries to shoot, it'd might miss him and blast open the door instead. Rick makes good use of his ability to scale the wall to get above it, dropping oil on top of it, and Khond follows soon after with the use of his tinderbox.

It's likely not an "abundant, exposed fuel" source... But as the fire gets lighted, you get the sense it's at least doing something. Not a lot, but something. 

Asimi hears Khonds statement about the gun, glancing up toward it... And decides to get closer to the frog thing, even if a bit hesitant.

[Damage detected. Aborting diagnosis.]
[Loading persona core...]

The creature's eyes flash as they switch to a sudden purple. The hollow voice that came out before suddenly sounds a lot more... Familiar.

[Finally! Let's start your combat assessment, shall we?]

The frog's stance has shifted, seeming ready to start acting now.

(24) Breldo <=
(23) Khond <=
(15) Rick <=
(12) Asimi
(0) Metal frog (burning)

Seeing the fire has had an effect Rick scooches back over top of the thing and empties his flask of lamp oil on the frog thing, trying to get into its head joints.

He will then move back to the wall/ceiling behind the thing.

Breldo will try for another shot at a less armored looking portion of the frog.

Attack: 12 Damage: 13

Breldo shoots a bolt from afar, but this time he's quite certain the shot misses simply due to the panicked shot.

More worrying is the part where the metal creatureimmediately reacts.

[Cowardice detected!]

Attack: 21 Damage: 20

The cannon on its back fires a devastating shot that strikes Breldo right in the chest! 

(24) Breldo
(23) Khond <=
(15) Rick <=
(12) Asimi
(0) Metal frog (burning)

Breldo has a moment of regretting going on the offensive before he gets blasted. 

"Fork you, frogface!"

((How's that fire going?))

((I'll let you know once Khond had his turn! Right now it's still burning, though it's not a massive fire or anything like that.))

Deciding stabbing and shooting clearly don't work - and since Rick's added oil should catch from the existing small flame, Khond opts for the mythical counter to armour; Blunt force trauma.
In other words, he kicks it.

Hooves attack: 13
Damage: 2

Khond goes for an improvised attack, but unfortunately finds his attack glancing, uncertain if a more percussive maintenance might have effect where arrows and swords clearly were not. The metal frog, however, immediately reacts to the assault again. The whole figure shifts as if it's trying to move backward but soon realizes that's impossible, and instead one of the massive legs lifts up, right over Khond. It's positioning to strike where the centaur is standing!

Asimi: 7

Attack: 18 Damage: 10

While Rick tips over more oil over the creature, increasing the burn rate just slightly, but admittedly nowhere close enough to seriously hurt the damn thing, he retreats to behind it soon after.

Asimi glances up at the leg but seems uncertain what to do, and just takes some distance from the coming impact site, while that cannon, still trained on Breldo, fires again. Another potent blast of energy impacts him for 10 damage this time around.

(Breldo, I think that puts you at 0HP? Roll me a death saving throw if so please)

(24) Breldo <=
(23) Khond 
(15) Rick 
(12) Asimi
(0) Metal frog (burning) (aiming for Khond's position)

Khond looks up at the raised foot, face blanched, then grits his teeth.  "Asimi, make sure Breldo gets to safety.  It can only aim that thing so far down, try to find a safe middle ground."
With that, the centaur does his best to circle around to the leg opposite the raised one, hoping it can't stop him through its own leg, and kicks again.

Attack: 15
Damage: 6

(Pretty sure Breldo should be down, so in the interim...)

Khond prepares to move, but just before he can finally get to act again does that leg slam down on his position!

10

Khond takes some damage at the grazing hit. The frog's eyes seem to flicker as the shockwave of that stomp bounces through it.

(Checking if you want to keep your action unmodified?)

(24) Breldo <=
(23) Khond <=
(15) Rick <=
(12) Asimi
(0) Metal frog (burning) (aiming for Khond's position)

Welp, since there's no escape, Khond is going to the right wall before doing his kick instead.  Hopefully if it targets him again it'll be in a spot where others aren't in the range.

Khond's attack once again glances. And once again, the big metal claw is positioning to strike where Khond is currently standing.

(24) Breldo <=
(23) Khond
(15) Rick <=
(12) Asimi
(0) Metal frog (burning) (aiming for Khond's position)

((Can Rick quickly loop his rope around the thing's laser head and front, lifted leg so when it drops it takes the head with it?))

(It might be possible, but it'd probably be difficult. An acrobatics check? And you'd have to risk getting on top of it's shell too I think. It's now partly burning shell.)

((Rick will risk it.))

Rick jerks out his rope and attempts to loop it around the blasty head and then the stomping leg to help friend Khond.

Acrobatics: 10