Lex Luthor is required in all matches of challenge battle 2. As usual, challenge battle 2 is by far the easiest - the opponents are mostly normal bronze levels and the tier of your characters are not restricted: you can use your favourite golds and the opponents here has about 1/10 of the health of the battle 5 opponents. Even for extremely new players with only bronze cards, this can be of use for them as the 6th match grants 4,340 power credits.
If you don't have any version of Lex Luthor, there is no need to dish out big money for Lex Luthor/Krypto at this point - just get the bronze Lex Luthor/Prime or the silver Lex Luthor/Insurgency, both of whom have useful team buffs that would make bronze or bronze/silver only battles (1 and 3) much easier.
Unusually, this match contains Regime Nightwing, a silver card when the other fights after it contains only bronze cards, making it harder than other matches - but still very easy.
Challenge: Dampening Field!
Challenge: Power Surge!