Normally, a downside to Auras in Voltron decks is that if the creature gets removed, the Aura doesn't stick around
Tyranid Invasion is great early (since you usually have three opponents), Tyranid Prime is absolutely bonkers, one of the best cards in the deck, and Tyrant Guard is pretty good, offering protection to your board
Youll need to match the symbols of the attack cost to the Energy card, but any kind of Energy can be used for
Rikku, Resourceful Guardian allows you to force a creature through your opponents' blockers, as well as steal counters, and Lulu, Stern Guardian protects you by putting stun counters on your opponents creatures, as well as proliferating to keep them around
The Crab first got the milling ability based on the logic I explained above in Drowner Initiate