| carn |
Mine is ice troll summoner.
He gets due to extra class levels (CR4 monsters allow 2 extra levels, costing effictively 2 levels) 18 levels of summoner and hits greater aspect. Uses 2 points for fire and 2 points for acid immunity.
Rest pretty irrelevant, as undispellable regeneration and decent str scores will lead to quite useful char.
I checked it several times and did not find any rule forbidding immunity for a vulnerability. At level 12 he can already get fire immunity and will use spells to get acid resistance. Then already useful against non acid monsters.
Edit:
With monster PCs one is allowed normal point buy, so with 25 pt buy, ice troll starting stats are, assuming str+8, dex +8, con +6, int-2 and cha-4:
Str 22, Dex 20, Con 19, Int 12, Wis 12, Cha 12
1 increase in con, all others in cha + cha boost item -> access to all summoner spell levels, but sucking spell dc, but he is about boosting and summoning anyway.