“Creature enchantment” here means “Aura enchantment with enchant creature.”

Paying {R} to end the effect is a special action and can’t be responded to.

Flanking means “Whenever a creature without flanking blocks this creature, the blocking creature gets -1/-1 until end of turn.”

If a creature gains flanking during combat, that has no effect on any creature already blocking it. A creature needs to already have flanking before blockers are declared in order for its flanking ability to trigger.

Playtest cards aren’t legal for play in any tournament format other than Mystery Booster Limited formats. On the other hand, we expect they will spice up a wide variety of non-tournament games (as long as everyone’s on the same page about using them!).

Playtest cards use a modified version of game symbols, such as {T} and {W}. These modified symbols should be treated as the standard symbols during play.

For many playtest cards, you’ll need to make a generous assumption that basic game rules would be updated to allow them to work. The Mystery Booster 2 Playtest Card Notes section (reproduced here in individual Gatherer rulings) provides guidance for fitting these cards into the existing rules structure.