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.