Attacking will either flip a good creature or you'll probably end up losing yours
You could also go artifacts or enchantments, since there are also many artifact creatures and enchantment creatures, especially ones that would give you additional cost reduction
The creature stays tapped if its already tapped from attacking or activating an ability and wont untap when the new creature is put on top of the merged pile
Training Grounds is an incredibly strong effect that reduces the activated abilities of your creatures by two generic mana
Normally, The Sixth Doctor costs a bit of mana to cast, but with Amazing Spider-Man, you can get The Sixth Doctor down on the battlefield for only three mana, letting you take advantage of its effect during the early stages of the game
Step into a high-fantasy animal world and take on a new adventure