Citrus based paint strippers work very well, and they smell like citrus.
When stripping paint, sometimes it is required to do it in stages, apply stripper, scrape and repeat (maybe several times).
On paint that is very difficult to remove sometimes using a heat gun will help soften the paint so it can be removed. Sometimes you need to employ both methods to achieve effective results.
I would use the citrus stripper and see how it goes.
Can I use a heat gun on my kitchen wall?
If, as you guess, the wall is plaster over brick then a heat gun used carefully will not damage the wall. If it is drywall (Sheetrock) then you need to be very careful. Drywall is gypsum sandwiched between two layers of paper so once you have removed most of the paint you could scorch the paper if not careful.