Folks have already beat grah using BB spam teams essentially. copypasta of a guide i saw elsewhere....
http://www.reddit.com/r/bravefrontier/comments/2a8qch/official_grah_achievement_mega_thread/
Active entire battle
Angel Idol buff.
Single-target dark elemental attack that inflicts injury and weakness.
AoE fire elemental attack.
AoE water elemental attack.
AoE earth elemental attack.
AoE thunder elemental attack.
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100% HP
Guard on the first turn. DO NOT try using a fujin and Damage Reduction buff, it will not work.
Powerful AoE dark elemental attack:
100% of your unit's total HP if you don't guard.
55% of your unit's total HP if you guard.
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< 100% HP
On the 5th turn:
Debuffs your units by lowering their attack (wears off at < 75% HP).
Light attribute buff (wears off at < 75% HP).
3 turn heal over time buff.
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< 75% HP
Guard on 5 units while the 6th unit lowers Grah below 75% HP.
Powerful AoE dark elemental attack:
100% of your unit's total HP if you don't guard.
55% of your unit's total HP if you guard.
When to guard:*
http://i.imgur.com/7nGlJ91.png
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< 40% HP
Attack buff (wears off at < 20% HP).
Single-target dark elemental attack that inflicts paralysis.
Single-target thunder elemental attack that inflicts paralysis.
Single-target earth elemental attack that inflicts weakness.
Guard on 5 units while the 6th unit lowers Grah below 40% HP.
Moderate AoE dark elemental attack.
6,660 damage if you don't guard.
3,330 damage if you guard
When to guard:
http://i.imgur.com/TIMZ3Tf.png
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< 20% HP
3 turn heal over time buff.
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< 15% HP
Attack and defense buff (wears off at < 5% HP).
Light attribute buff (wears off at < 5% HP).
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< 5% HP
Defense buff.
3 turn heal over time buff.
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0% HP
If your current squad is your only squad or last squad remaining then heal at least one unit to 5,001+ HP and guard, while all remaining units lowers Grah to 0% HP.
Powerful AoE dark elemental attack:
9,999 damage if you don't guard.
5,000 damage if you guard.
Grah is now immobilized, so finish him off.
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