{Blood Bairn}'s ability + {Gravecrawler}'s ability + a zombie on the battlefield & enough mana to do it. Is it possible to sacrafice Gravecrawler w/ Blood Bairn's ability over and over and over and stack up on +2/+2 counters as long as you have the mana to keep bringing gravecrawler back?
Yes it is legal
To be sure, {Blood Bairn} gets +2/+2 until end of turn, not +2/+2 counters.
And since you can only cast creatures in your main phase, it can only get to its biggest during your main phase.
The combo works the same if you use {Tenacious Dead}. The combo costs a little more but it only requires two cards ({Blood Bairn} and {Tenacious Dead}) and will be legal longer. A few guys from my LGS figured it out during the prerelease and dominated with a G/B aggro.
Works quite well with grimgrin although he's not legal long