ok you mean post #224.
This zombie has a very high health points, (over 10K and that's more than twice that of the big garg) So try to imagine you need the fire power to kill 3 big gargs to kill him and you will need to do this 3x on the 1st appearance, 4x on the second appearance and 5 times on the last appearance in a game to earn his picture in the Almanac.
As for which plants are best to use, this depends on what you have, generally, all the insta-kill plants, for example explosives, squash, Jalapeno etc and all your high damage dealers plants such as Magnifying Grass and Citron etc... or better still enhanced by mints.
I copied the table below for reference but you can read up in more details from the link
https://plantsvszombies.fandom.com/wiki/Roman_Caesar_Zombie
EDIT: correction to the number of times required to kill the zombie.and added the plant suggestions.