I have got a few decks which I use for ELO, some with success and mixed results with others.
Now I'm gonna list a few of the decks I have used, any tips/suggestions or comments would be appreciated even if someone else has already said it!
Half-decking is easier, IMO. In Mono you will find that each clan has a weakness of some kind... Too many low-damage draws vs SoB or SoA, not enough round-winning power, etc. Playing with 2 clans offers the opportunity to create a deck with more well-rounded draws. However, it's best if your cards can survive without bonus.
P.S. Phyllis is a 3* in a 2*s body that doesn't require revenge. Compare to Praxie (she may not have min 0 like he does, but the power difference and higher min is compensated by the more versatile SoB, which also protects ability vs GHEIST and Roots.)
Phyllis is a really nice 2* but its utility drops towards the end of the battle (So both Mawpin and Phyllis have a certain extent of restriction on their usage, maybe Phyllis has less). But I agree with ghelas that she is a really good choice for a NightmareELO deck. It's a matter of preference.