Combos:

infinite +1/+1 counters: counter doubler (hardened scales, vorinclex, doubling season, etc.) + Rite Of Passage + Walking Ballista with 3 or more counters. Remove a counter and target ballista, rite of passage and counter doubler puts two counters back on ballista. caution since ballista will have a lot of damage marked on it.

deathtouch machine gun: Walking Ballista + Basilisk Collar + Blade of the Bloodchief = kill all creatures

Kami of Whispered Hopes + Umbral mantle = infinite mana

Legolas's quick reflexes + Seeker of Skybreak = kill all creatures

ACR - Cleopatra, Exiled Pharaoh - At the beginning of your end step, put a +1/+1 counter on each of up to two other target legendary creatures.
                                                        Whenever a legendary creature with counters on it dies, draw a card for each counter on it. You lose 2 life.

BLC - Thickest in the thicket -  When ~ enters target creature gets X +1/+1 counters where X is its power. At your end step if you control most powerful creature draw 2 cards.