Please consider:
- Heavily furred or thick hided animals or summons should have a point or two of damage resistance.
- Heavily furred or thick hided animals or summons should have a point or two of cold resistance.
- Agile / many-legged animals or summons should have a stronger reflex save + discipline bonus. By way of example, at an early level, a summoned spider seemed unusually prone to falling over via a Grease spell.
Having a base level of the above would be a minor tweak that would aid summons longevity a little bit from mid towards upwards PC levels. The tweak would aid in supporting the idea that such animals are the fittest, premier examples of their breed, toughened by their environment.
Edit - Spell Focus: Conjuration:
I've not tested Spell Focus yet, but please consider a further minor tweak via what benefits GSF: Conjuration brings. That is:
Creature Summoned is, in terms of hardiness, best of Breed + Spell Focus represents training / taming (enhanced AB, endurance, saves).
Summons and Animals Tweaks
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