VECTOR temp,temp2;
...
vec_diff(temp,ent1.x,ent2.x);
vec_to_angle(temp2,temp);
ent2.pan += ang(temp2.x-ent2.pan)*0.1*time_step;

Try this. If the entity is turning too fast, replace the last line with the following:

ent2.pan += clamp(ang(temp2.x-ent2.pan)*0.1,-5,5)*time_step;

and adjust the max. speed by replacing 5 with different values.