Total War: NAPOLEON - Definitive Edition

Basically, you should play on the easiest difficulty, only autoresolve your battles, and only use three armies with the same generals. Don't use well balanced armies. Use armies that are heavy with one type of unit. For instance, for Napoleon i'd give him almost all artillery and just a couple infantry units. Doing it this way allows the general to gain stats to fighting with certain types of units such as artillery, cavalry, and infantry.

These are all doable on Multiplayer only, as the Europe map is only available there.

Description of this achievement says it all mostly as to what country you have to win as.
Find a boosting partner and work together to trade wins with nations.

If you play naturally without using "Autoresolv" this comes pretty fast. Kills done with autoresolv do not count towars this.