Ichis are best used as a tank, even when you get Crabatron. Ichi is one of the best monsters that attack defense towers because they take less production time than Crabatron, and are statistically better. Making 10 Ichis will give you the same power as 5 Crabatrons at level 1, but when Ichi reaches level 2, his health per housing and damage will be higher than Crabatron. Ichis are great to use in groups.
A bunch of Ichis attacking a defense tower at one time can destroy the tower within seconds. Ichis are the best way to soften up defenses and then send in a Champion to loot the enemy yard's resources at lower levels.
You can use Ichis to make the enemy champion go to that side while your monsters are looting resources.
Ichis are also good in bunkers, as they take up less space in bunkers than Crabatrons and are cheaper. They can help to tank the damage dealt by enemy monsters while high-damage monsters do the damage. Multiple Ichis, when combined with a Fomor, can take down an outposts easily. If attack a main yard, it is best to include a couple of zaffretis so they last longer.