Using "Mystic Box", you can destroy your opponent's strongest monster and inflict 2000 damage at the same time. Then attack "Ameba" with one of your monsters for more damage.
After giving "Ameba" to your opponent (inflicting 2000 damage in the process), you can equip it with "Mist Body" or "Heart of Clear Water ". On top of that, you can activate "Shadow Spell" that way it will be unable to change its position or attack. Then, attack it with all the monsters you have for potentially huge damage, possibly even enough for an OTK. "Half Shut" can work to with this combo instead of using "Mist Body" or "Heart of Clear Water".
You can reuse "Ameba's" effect by Special Summoning it from your Graveyard with "Kinka-byo" or by returning it to your hand with "Swap Frog's" effect. You can combine both of these cards to get "Ameba" out on the field by sending it to the Graveyard first with "Swap Frog", then Special Summon it with "Kinka-byo".