While the 7 int for the eidolon does not seem that well if you take the skilled evolution for +8 on the targeting check with knowdge egineering. then put ranks in it and make it a class skill and you are hitting on a 4 without modifiers with a bonus of to hit of +11 from skilled evolution and ranks in the skill. You will need exotic weapon proficiency light catapult though. Of course it should be bipedal to get hands. If the gm allows a +5 knowledge eginering item probably goggles it gets even better. Maybe take skilled perception as well.
While the 7 int for the eidolon does not seem that well if you take the skilled evolution for +8 on the targeting check with knowdge egineering. then put ranks in it and make it a class skill and you are hitting on a 4 without modifiers with a bonus of to hit of +11 from skilled evolution and ranks in the skill. You will need exotic weapon proficiency light catapult though. Of course it should be bipedal to get hands. If the gm allows a +5 knowledge eginering item probably goggles it gets even better. Maybe take skilled perception as well.
I'm not 100% sure what the intent behind a build like this is, but I'm chiming in because we recently had to deal with a siege in our campaign and my Summoner (8th level Halfling Master Summoner actually) really got to shine. The character for RP purposes summons only elementals. Hie eidolon is a little pocket rogue, serving as a scout with flying and all of the appropriate Skilled evolutions, buffed with an Invsibility spell as often as not.
Anyway, My character was able to summon either 2-4 Augmented Medium Earth Elementals (can Burrow 20) or 2-4 Augmented Medium Air or Lightning Elementals (Fly 100) 13 times a day that lasted a good 8 minutes or so each time. That was more than enough to wreck havoc on the town and bring them to their knees, especially if I decided to buff them with things like Hasten.
No siege engines required.