How to win in 100 words or less? Very simple. Just eliminate ALL opponents' armies from the map and you are the winner.Waterloo is one of my favorite maps because it has some interesting perks, like artillary, calvary, a major obstacle dividing the map and an assortment of bonus options. You have to pay attention to what gives bonuses and what territories are most vulnerable here... notice that all territories have red/blue signifiers that designate where bombardments can strike. There are only a handful of territories plus Blucher that artillary can't touch.
Your Game was a 1v1 with your opponent (red) going first, which adds to your dilema and on top of that they start with extra armies for holding Chasse. Red deploys 13 in Round 1, knocks you off a few territories and captures some flags to add +3 to their bonus. You (green) start already behind and receive 9 armies which you proceed to sprinkle all over the map ignoring Red's bonus, which for Round 2 is 16 to your shrinking 7. One way you could have struck back? Deploy armies on West side blue artillary to
break Chasse and wreak as much damage to Red territories and bonus as possible. The nice thing about artillary when you're outnumbered is the simple fact that you do not have to occupy territories you defeat (they become neutrals) which leaves you more armies to press the attack by bombarding elsewhere. The downside of artillary is all armies deployed there cannot be forted to help elsewhere. Green is now full of holes and sinking as Round 3 presented Red another 16 deployment while yours had shrivelled to 4.
Ignoring where opponents' bonus armies come from is rarely a good survival strategy. Round 4 and you're toast as Red gets a 20 bonus... and reduces green to a 3 bonus with 3 territories.
Round 5 and you're eliminated, but this one was really over at the end of the first round.