Thinking of how to do Murderbeast Distraction, Mathilde could slip into the pit below under Substance of Shadow, then use her shadow chisel and greatsword to shave off pieces of the wooden floorboards and supports in such a way as to weaken the tie downs and its ability to resist the monster jumping through it. That should also produce a pile of kindling. IF she can do so while keeping the fire hidden (such as a piece of cloth, and alcove, or appropriate stone), she can then use the kindling to start a fire in the pit, ideally somewhere it will spread and eventually light the floorboards, while Mathilde bugs out back through them before the fire grows enough to illuminate the pit. The combination of smoke and flame should make the monster very keen to get out.
Another option, rather than a fire, is to leave the pit after memorising where the monster is and weakening the floor, and then fire a Substance of Shadowed gun/bullet through the floor at the monster. That should piss it off enough to jump through the weakened floor, and which point Mathilde can then set the resulting shattered wood in the pit on fire. The smoke that produces should help her remain unseen and distract the goblins further.
Yet another option might be to cast burning shadows on the shadows cast by the goblins and the bolt throwers, tailored so it only affects the bolt throwers the wooden floor, and the monster beneath. The goblins won't know what's happening, but the bolt throwers string will gradually be dissolved by the acid by the shadows of the goblins, and the shadows of the bolt throwers will gradually dissolve the floor and then start hurting the monster when that gets some holes in, causing it to get pissed off and jump through the partially dissolved hole.
However, I'm going to go for:
[X] Plan Everything is Melting!
[X ] Sabotage the defences...
- [X] Other (write in)
- [X] Cast Burning Shadows on each of the two bonfires, excluding yourself, dwarves, humans, halflings, winter wolves, and demigryphs from the effect. The shadows of the goblins will melt other goblins, causing pandemonium, and also melt the bolt throwers, the shadows of the bolt throwers will melt the wooden floor, and as that gets holes in will start to melt the monsters, which should psiss them off and cause them to jump out and add to the havoc.
[X ] Slip out...
- [X] By walking out while they're otherwise distracted. (note: ensure your plan includes an otherwise distraction)
- [X] By casting Doppelganger and Take No Heed, and just walking out. Goblin crowd is thick enough to probably require some shoving.
I suppose this technically qualifies as Murderbear Distraction in a tenuous way, as it does involve releasing the murderbeast.