I like the dimensional bolt concept. You'll still have to work with other skill trees/characters to get more reliable surfaces and elemental interaction you want, but summoners should have some way to create a basic surface, even if it's random.
There is no scenario in which this skill is a necessary part of the Summoning tree.
You have 3 other characters, grenades, barrels, arrows, enemies and the almost excessive prevalence of environmental surfaced, to rely on for surfaces already.
On it's own it's a skill that could be interesting, but as part of that specific kit it's just superfluous and misplaced.
