Below the command which displays the requested transformations.
$ ex list.txt +'1,2d' +'g/^break$/d' +'%s/this//g' +'wq'
Arguments of the given ex command:
From line number (1) until (,) linenumber (2) inclusive, delete (d) line:
+'1,2d'
- Delete a specific line.
For all line from beginning-to-the-end-of-file (g) match on starting line (^) with text (break) followed directly with end of line ($), do delete (d)
+'g/^break$/d'
- For all lines (%) subsitute (s) matching (/) something (this) (/) [with nothing] (/) also if multiple times one a line (g)
+'%s/this//g'
- Combined with the ex commands to write and quit:
+'wq'
INPUT: list.txt
this line 1
this line 2
this line 3
this line 4
break
this line 5
this line 6
RESULTING OUTPUT: list.txt
line 3
line 4
line 5
line 6
Good luck