At the declare blockers step, all 11 are declared as blockers for breaker of armies. Next, the attacking player gets to decide the order in which the blocking creatures block. In this case, since all the tokens are identical, it doesn't matter. Then each player has a chance to cast instants or use abilties. Assuming neither does it, we go to the combat damage step.

Attacking player has some (most of the time straightforward) choice in how to apply damage. Starting from the first defending creature (that was chosen earlier), we must apply "lethal" damage before we can do anything to the next.

Lethal damage is, in most cases, equal to the toughness of the defending creature. Occasionally it is different, for example a creature with deathtouch can deal only 1 damage to each creature.

The total damage assigned will equal the total power of the attacking creature. Since breaker of armies has 10 power, his damage will be assigned as 4 to first plant, 4 to second plant and 2 (non-lethal) to the 3rd plant. That is all the damage breaker of armies can do, but that does not stop the plants from dealing their damage. Each and every one will do it's 3 damage for a total of 33 damage to the breaker of armies. So in the end 2 plants and the breaker of armies will die.

An important note is that once both players chose not to cast their instants they don't have a chance again until after combat damage is applied. I.e. you can't buff the first plant AFTER four damage has been assigned to it to keep it alive.

Most of the time this step is straight-forward and you won't have to think about it, but occasionally it will matter. For example if the attacking player had a breaker of armies, but defending player has a 1/1 token and a [[hornet nest]], blocking the token first means that the entire 9 remaining power of breaker of armies would be dealt to the hornet nest, spawning 9 insect tokens. But if the nest was blocked first only 2 damage must be applied to the nest, spawning only 2 tokens.