If a creature is damaged, note how much damage it took, in case it suffers more damage later in the turn (you usually won't need counters for this)
I'll be posting my list soon
For the cost of seven mana, this Slug Horror states that at the beginning of each player's upkeep, a slime counter is put on each creature you don't control, with each creature getting -1/-1 for each slime counter on it
Its important to keep expectations in check, and to understand that it takes a lot of work and research
As Commander is a format rife with mana rocks and utility artifacts, this single card can most often provide its caster with enough mana to run away with a game, whether it's played in the early or late stages of a Commander game
The one way to break it's cruve-based drawback is to have your turn two play be a ramp spell (like Growth Spiral, Incubation Druid, Paradise Druid) which in turn allows you to play a four-drop on turn three