Mephidross and Metallic Mimic

[entropize]

Alright I think this combo would work but I am only about 75% certain it works. Anyway here is my question, Say you have Mephidross Vampire in play and metallic mimic in play with the chosen type being vampire. Now say you bring out walking ballista, does it get an additional +1,+1 counter since mephidross is out which makes all of your creatures vampires and metallic mimic is out? I am worried that since both effects trigger at the same time, there might be some dispute about this combo.

[CardAgain Sweater]

I think that works.  If you have those 2 on the battlefield, and the balista resolves its an artifact vampire. Why wouldn't it?  Nice combo Some unblockable aura or - gasp - a slither blade (or dimir infiltrator) if blue is possible would be sweet.

EDIT: I read that card wrong. You want to do damage to a creature. Ignore my silliness, the answer below this post is what you want.

[Jabilac]

Mephidross Vampire type changing text is a static effect and is being continuously applied.  In your described case Walking Ballista will enter the battlefield as a Vampire Artifact and will ETB with 1 additional +1/+1 counter because of Metallic Mimic.  It will gain a +1/+1 counter when it deal's damage (Not just combat damage) to a player but you have to let the trigger resolve before you can get the +1/+1 counter and if it dies before the trigger can resolve then it will not get the counter.

Example:  Walking Ballista with 1 +1/+1 counter will not be able to survive if you remove the 1 counter because State Based Actions will have it put in the graveyard for having 0 toughness.  Walking Ballista with 2 +1/+1 counters will survive as long as you do not let its toughness get below 1.   In this case it would be:  Remove a +1/+1 counter to deal 1 damage to your opponent, after the damage is dealt, you will get a trigger that says "Put a +1/+1 counter on Walking Ballista" you have to allow that trigger to resolve before your Walking Ballista will get that +1/+1 counter.