For a Summoner?
Tower Shield Proficiency (although it's not technically needed, as you're not planning on making any attack rolls or anything).
You see, Summoning isn't an attack, and so isn't given up by using a Tower Shield for Total Cover - so you can be immune to missile fire while summoning up beasties. Find a way to stay out of melee range, and some way to deal with spells, and you're golden.
I'd suggest checking with your DM to see if Natural Bond would mitigate the level-adjusted animal companions in your campaign, but you dropped the Animal Companion, so it doesn't much matter; with Combat Riding, Tower Shield Proficiency, and (advanced animal companion mitigating) Natural Bond, you can be constantly flying on a tough mount (via a Dire Bat with full bonuses due to Natural Bond - even in medium dungeons, if you're a small character - a Druid-4 has access to Reduce Animal) while being yourself basically immune to mundane attacks while summoning beasties to lay the hurt down. Tack on Augment Summoning and Greenbound Summoning and... well....
Oh, and there's a sharper limit on Wall of Thorns with the Greenbound Summoning trick - you see, any effect with a non instantaneous duration produced by a summoned critter ends when the summoning spell does. As it costs the critter an action to actually put up the wall, your first level druid having the greenbound summoned critter put a Wall of Thorns up doesn't do much... as (depending on interpertation) the summoning spell ends just after the beasty actually gets to take it's action. At 2nd+, on the other hand....