Use 2 cerberus and 3 Union attack. Play against lvl 10 lds. I used to have 2 cerberus on field then increase their attack to 10k+. 3 Union will give you 65k attack. You can use spell reclaim to get even further but it will only count as 65k
I have just been doing it with Odion Chain Reaction skill, some traps, some burn/heal like goblin thief and Abare Ushioni. Generally only against level 10 and level 20 gate duelists but if you want to level up Odion at the same time, set for level 10, x3 multiplier, get lots of chests for rare farming and 1500xp per go. Makes it slightly more profitable and less boring.