Balancing Achilles Heel of A Construct (PEACH)

[DracoDei]

Got a construct I am working on... wondering about appropriate levels for it. It has an Achilles heel that negates its Spell Resistance and Damage reduction under the right conditions and possibly the energy resistances as well... wondering what people think the CR should be both with and without those conditions:

????
Medium Construct
HD 3d10+20 (Average 36 hp)
Speed 20 ft. (4 squares);
Init: +0
AC 14 (+4 Natural); touch 10; flat-footed 14
BAB +1; Grp +5
Attack Slam +5 (1d6+3)
Full-Attack 2 Slams +5 (1d6+3)
Space 5 ft.; Reach 5 ft.
Special Attacks
Special Qualities Woodland Stride, Trackless Step, DR 10/-, Fire Resistance 5, Electricity Resistance 5, SR 20(unless specific conditions met), Tree Shape(Oak Only, size equal to that of the ????) 2/day, Construct Traits
Saves Fort +2 Ref +1 Will +0
Abilities Str 16, Dex 10, Con -, Int 14, Wis 5, Cha 1
Skills Move Silently
Feats Iron Will(B), Great Fortitude(B), Stealthy, Alertness
Environment Deciduous Forest
Organization Solitary or Pair
Challenge Rating 5? (but see "Usage" below).
Treasure None
Alignment Always Neutral
Advancement 4-8 (medium); 9-20(Large);21-30(Huge)