If you're serious about challenge and don't want the AI to be cheating, try
Carrier Battles mod
If you're stomping all over TDM AIs with 5x bonuses, I suggest playing team mode, NO bonus.
Non-team mode should be good to let you learn the ropes (rule one - make friends with the Vikings
)
If you want the AI to steamroll your empire and then capture your homeworld with troops within the first 75-100 turns, try low bonus
