The caster starts a battle casting blink upon himself.
On his next turn, he uses ready action: he will cast banishment on himself while he is in the ethereal plane, thus returning to the material plane, and not losing concentration.
When his next turn starts, he is banished from the ethereal, holding concentration on banishment– he let go of blink, using his action, and let go of banishment concentration as a free action- since banishment was stoped before it ended, he is brought back into the ethereal, but without means to return (plane shift spell would be a returning option).
So this way you have a character trapped on the ethereal plane, and it can be achieved with a 3rd and 4th spell slot.
The question is: is there any sort of offensive device that could be used from the Ethereal to the Material Plane?
It has to be something a 7th level character could use, since this scheme can be used only as early as that.
Being trapped into the ethereal is not so fun, but if you could use a spell that would hurt people on the material plane, plus you have sending as a way to communicate with your allies. It only lacks an useful offensive tool.
Best Answer
Sort of
When you are trapped in the ethereal plane, there are no official ways to damage enemies in the material plane, but there are some spells that interact with both planes at a given time. From Appendix C of the basic rules, under the "Transitive Planes" heading:
It stands to reason that one of these spells cast on the Ethereal Plane would extend into the Material Plane.
Homebrew Options
In my current campaign I've been GM'ing recently my players got stuck in the ethereal plane and I gave them the option to construct devices to allow for certain effects to transfer over (mostly force damage spells). One of my player's artificer constructed a magical pseudo-Gate that allowed for only force spells to go through. It is mentioned in the description for the Ethereal Plane that it would be easier for effects that use force energy to extend into the plane so it seemed reasonable to allow for an option less than a full gate.