from http://dota2.gamepedia.com/The_International_2017_Battle_Pass#Path_of_the_Wave_Breaker
it reads:-
Purge 5 / 10 / 15 debuffs using Manta Style
can you use something like glimmer cape, and then manta to complete this quest without relying on something else doing something to you?
No.
These "self-effects" are generally good and are called buffs, not debuffs. Buff and debuff icons are shown above the character health bar. Buffs (friendly effects) are shown in green, and debuffs (enemy effects) in red. As described on the wiki:
Dispels remove status effects based on whether the casting unit is an ally or an enemy, and whether the status effect is positive or negative. Applying a dispel on an allied unit only removes negative effects (also known as debuffs). Positive effects (known as buffs) are never removed.
There may be some exceptions I'm not aware of.
What you can do is find a group of prowler ancient creeps. The large prowler shaman reliably casts two debuffs (a root and negative armor) which can both be dispelled with Manta Style. The root doesn't even cost mana, so a single camp will do if you don't kill it. I did this yesterday to achieve the first star near the end of a game.
This should also work with the vhoul's poison and the ghost's slow. The hellbear's thunder clap slow and Roshan's slam slow can be dispelled but both of those need 3 enemy units nearby to cast. The small satyr's purge slow, the ogre frostmages's frost armor slow, the troll summoner's net root and the thunderhide's slam slow can be dispelled, but neutral camps will never cast these spells.
