"metamorph with a spear" is actually easy to build. because "metamorph" is not really a class.
so you start with metamorph preset(to get the spear in the chest), but put all points into Finesse.
I suggest to take opportunist talent if it's not selected by default.
then as you level up you add 1-2 points into scoundrel to get useful skills like Adrenaline\Cloack and dagger and put most points into warfare to increase damage of your spear.
as for polymorph skill since you lack srt stat to make it damaging you should select skills mainly for utility: chameleon, heart of steel, wings and etc. and even tentacle or medusa can be useful for their effect even though they'll do almost no damage.
the rest of the points and talents can be spent on everything you want. necromancy+living armor if you wanna pretend that you're tanky. geo\hydro\air\fire for useful buffs and maybe even CC spells to help your party. and etc.
