Assault is a Bravery deck specific effect card. By paying 1G, it will add 3 random monsters with a cost of 3 or lower to your deck and they will get +1 ATK and +1 HP. It also lets you draw a card afterwards.
Though you don't know which cards you get, you should have at least 2 of these in your deck, as it increases your deck count which helps the heavy draw class Bravery is. It's also argueably better than Strength due to the fact that it also gives +3 ATK and a +3 HP bonus, but split among the 3 monsters you obtain for 1G less.