How to beat stronger players
How to beat stronger players? If you are massively out-rated, the simple advice that I recall is to try to really complicate the game, if that is at all within your style. Otherwise, normally play your best stuff against the big boys, as far as opening choice goes. Perhaps other posters may add to, or debate, this advice.
In the game below, things went like a dream for me when FIDE CM David Gordon tried his (of late) usual Modern Benoni Defence. I chose one of the best systems against it, starting at move 9. A dubious response, followed by a slight slip, was all it took for Black's game to begin going downhill:
[Event "RA Winter Frost"]
[Site "Ottawa, CAN"]
[Date "2017.12.7"]
[Round "3"]
[White "Pacey, Kevin"]
[Black "Gordon, David"]
[Result "1-0"]
1.d4 Nf6 2.c4 c5 3.d5 e6 4.Nc3 exd5 5.cxd5 d6 6.e4 g6 7.h3 {This move, or first 7.Bd3, is a little-known/used slight refinement to discourage ...Bg4 [which might soon follow if 7.Nf3]. Black seems to have little better than to transpose to the main line position that's reached at move 9 of the game continuation.} Bg7 8.Nf3 O-O 9.Bd3 b6 {This little-used move has a very poor reputation, but maybe it's not too bad.} 10.O-O Ba6 11.Bf4 Re8 {All ECO gives is 11...c4 12.Bc2 b5 13.a3 with a claim of big edge for White.} 12.Re1 c4 13.Bc2 b5 14.a3 Qb6 15.Be3 Qc7 {Better than this natural-looking move was 15...Qb7, with only a slight plus to White according to my engine.} 16.Nd4 {Now White has a huge edge.} Nbd7 17.Qd2 Qb7 18.Nc6 Nb8 19.Nb4 Nfd7 20.a4 bxa4 21.Nxa6 Qxa6 {It was better to take back with the knight. Now White is winning, says my engine.} 22.Bxa4 {The game goes on to move 51, but to my pleasant surprise my engine only quibbles slightly with some of my choices the rest of the way, starting here - it slightly prefers 22.Rxa4. One moral here is an old observation that if Black makes a mistake in the Modern Benoni, he's usually just dead, compared with many other openings.} Qb7 23.Nb5 Rf8 24.Nxd6 Qxb2 25.Qxb2 Bxb2 26.Ra2 c3 27.Nc4 Nb6 28.Nxb2 cxb2 29.Bb3 N8d7 30.Rxb2 Ne5 31.Rbb1 Nec4 32.Bxb6 Nxb6 33.e5 Rfe8 34.f4 Kf8 35.d6 h5 36.g4 hxg4 37.hxg4 Rec8 38.e6 fxe6 39.Rxe6 g5 40.Rg6 Nc4 41.d7 Rc7 42.Rxg5 Rd8 43.Rc1 Rdxd7 44.Rxc4 Rb7 45.Rc8+ Ke7 46.Re5+ Kf6 47.Rf8+ Kg6 48.Rg5+ Kh6 49.Rf6+ Kh7 50.Bc2+ Kh8 51.Rf8 {Checkmate.} 1-0
There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy.
Hamlet (1.5.167-8), Hamlet to Horatio.