Started by going around the outside anti-clockwise , hugging the edge and trying to take out no more than a 2-wide line and moving diagonally at corners to hug the edge. Left most of the tar intact in the SE corner and on the north side, at first.
× Had to cross over (stepping diagonally as far as possible, to leave gaps) before traversing the gap between (6, 11) and (10, 12), into the SW corner: because, when going into that SW corner, it is impossible to leave the right shape of tar to clear the whole lot and escape if you approach on the west side. (I needed to leave a 2-wide blob on (7,10) and (8,10) by clearing (9,10) from standing on (8,9): then leave a 3-wide blob below it from (7,11) to (9,11), again clearing (10,11) from the diagonal: then stand on (10,11) and turn to take out (9,12), to get in - then cut back over to the western side, hit the savepoint and once again approach round the outer rather than inner edge.)
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× After that, the main difficulty in the SW corner was being able to get in and then back out again while cutting out the tar, and leaving enough routes in and out in a small space, given the extra objective of dropping all the trapdoors.
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× Eventually what I left was an 8-shaped blob of tar (7 tiles) next to the southeastern bomb, on the wood floor, for the bomb to take out.