I decided to put together a summoner, and the first reaction that I saw was that the eidolon was rather broken, so I decided to do a little test of the creature. Neither the eidolon nor the summoner has any items, and it is assumed that the summoner is staying within the prerequisite 80 of the eidolon invisible.
The setup is that the summoner and the eidolon is starting 150 ft away from the frost giant, and the frost giant is up a gentle hill that covers the last 50 ft (treated as difficult terrain). The frost giant also has an ample supply of rocks.
Other assumptions: Huge evolution actually gives 15ft reach, as well as it's other listed effects.
Eidolon
10d10+50
105hp
Base form Biped
Str 36
Dex 10
Con 21
Int 7
Wis 10
Cha 11
Free Evolutions
Claws (1d4, arms)
Limbs (arms)
Limbs (legs)
Natural Armor +2(base) +2(Large) +3(Huge) +4(INA)
+8(11th summoner)
AC 27 FF 27 Touch 8
Base speed 30ft
Saves
Fort +7 +5 = 12
Ref +3 +0 = 3
Will +7 +0 = 7
+23 Bite 3d6+19 damage + 1d6 electricity damage
and 1d4 Str 1/rnd for 4 rounds or 1 successful save Fort DC 22
+21 2 Claws 1d8+6 damage+ 1d6 electricity damage
Feats (5)
Improved Natural Attack (Bite)
Ability Focus(Eidolon poison)
Power Attack
Cleave
Great Cleave
Evolution Pool = 15
Large(3)
Huge(4)
Bite(2) (1.5 str)
Poison[Bite](2)
Improved Natural Armor(2)
Energy Attacks(electricity) 2
Test 1: Eidolon wins initiative.
Round 1
Eidolon moves 60 ft towards the frost giant [90 ft away from giant, 40 ft away from hill]
Frost giant throws a rock at the eidolon (natural 20, 9+1 does not confirm) 16 damage, eidolon has 89hp left
Round 2
Eidolon double moves to close, nets 50 ft [40 ft away from giant, 10 ft up the hill]
Frost giant chucks another rock (9+5=14 miss), then draws his greataxe.
Round 3
Eidolon charges, ends 5 ft outside of the frost giant's reach, and attacks (25+18=42 vs AC 21) 32 total damage (2 of which is electricity)
101 hp remaining Fort save 14+16=30 vs DC 22, is not poisoned)
Frost Giant 5ft steps toward the eidolon, then full attacks with the greataxe
18+3=21 vs 25 fail
13+10=23 vs 25 fail
Round 4
Eidolon full attacks
23+3=26 vs 21 28 damage (2 electricity) 73hp and 14+9= 23 vs DC 22 not poisoned
21+11=32 9 damage (2 electricity)
21+17=38 18 damage(6 electricity) 46hp on the frost giant
Frost Giant full attacks
18+5=23 vs 27 fail
13+18=31 vs 27 23 damage 65hp remaining
Round 5
Eidolon continues to attack
23+7=30 32 damage (3 electricity) 14hp and 14+4 =18 vs 22 for 4 str damage
21+5=26 15 damage (1 electricity) -1 hp
In the end, the eidolon still has more than half of his health, and the summoner used 1 second level spell (invisibility). I also don't think that the build was not the most optimal. I don't suspect that having the frost giant win initiative would have made as much of a difference, and both sides power attacking would have ended even worse for the giant. I'd hate to think of what would happen if the summoner and eidolon were properly equipped.