A more concise step-by-step
1) create an account of the alternate faction
2) get the rep of the two accounts within 2500 of each other
3) On your defending city, Set the commander to your highest level hero.
4) On your defending city, change your defense to []Anti-Scout [x]Counterattack, with no troops set.
5) On your attacking city, send your lowest level hero with 30 marksman attached in one group. (don't split the 30 up)
6) When combat arrives, kill the 30 marksman and then move away from the other hero so they attack the wall.
The hero will not be able to destroy the wall within 30 turns by themselves and when combat ends, you should get a huge boost in rep.