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stompy: feeder, morph, weaver
ankh sligh: woodripper, morphling, feeder
other sligh: (i stick with dryads, so i can't help ya)
super gro: feeder, weaver, morphling (if they run scroll racks or something else, maybe ripper)
mask: woodripper, weaver, morphling (and yes, this works quite well)
tnt: woodripper, weaver
white weenie: morphling, weaver, feeder
psycho-gro: weaver, morphling
sui: weaver, feeder, morphling
Stompy: I don't know that I'd even bother with Feeder, here. Be aggressive in finding an Oath, and that'll grab you a nice Morphling/Weaver. Usually you'll drop your feeder for your sideboarded 2nd Weaver, here.
Ankh Sligh: Morphling, Feeder, 2nd Feeder/Baloth is the norm. Since you have alternate ways of dealing with everything else, counters and kegs are usually enough to take care of Scrolls/Ankhs to give you time to kill.
Other Sligh: Nothing to it. Swap out Weaver for Baloth.
SuperGro: You can either take out the Feeder for whatever other anti-gro card you have, or replace it with the second Weaver. Your call depending on if you want to see Weaver half or two-thirds of the time.
Mask: This is the most likely matchup for me to remove Morphling against. Although Morphling against a Negator is wonderful, Morphling against a Dreadnought could spell a crucial loss of a significant amount of life. Weaver and Woodripper go in, Morphling is your call.
TnT: Morphling, Weaver, and Woodripper. All do well against fatties.
White Weenie: Morphling and 1-2 Weavers is all you need, here.
Other Gro: Morphling and 1-2 Weavers.
Sui: Morphling and Weaver should stay in. If you have Triskelion/Hellion, bring it in over Feeder to make them pay for their use of Negator.
BASICALLY:
-You take out Feeder against decks w/o a lot of fast damage.
-You take out Weaver versus decks that are likely to remove it before it's useful.
-It's pretty damn rare that you'll take out Morphling, but it is done, sometimes, in a gamble in hopes that two more bullet-like creatures will just own the board.
Before TnT, Mask, and Gro became so popular--back when I was running 5cOath--I used to side out Morphling against pretty much everything that wasn't control or didn't have StPs. Shivan Hellkite nuked all opposing wieners and killed in 4 turns--so what if it got double bolted, it would be back. Maindecking it is not a problem, since it is able to block an opposing Morphling, unlike Triskelion/Crater Hellion/Ancient Hydra.