I propose a new tool useful until the endgame, but not ridiculous in power: The Tome Of Seperation!
You read that you could have 1 sentry take 2 sides with that? But how would you acomplish that when it would just split your sentry into 2 at the same spot? With this: The Etherian grabber!
@hogndog is a summoner, so lets SUMMON him here because he probably understands this stuff
It replaces books in the dungeon at the same rate as the water bolt
When used, all minions out will split into 2 with half the damage. This will work on sentries, but not the stardust dragon.
You could use this to hold a hoard now that you have twice the knockback, or have sentries each take a side in an invasion. But they would less effectively get through defense.
Sold for 10 gold by the tavernkeep
Picks up and moves sentries (OOA and regular)