When playing Agravain, it is absolutely necessary to be able to pull off the megablast. His megablast is easily one of the most powerful in the game, since it's an ACT skill. His megablast allows you to have a permanent +5k power boost when you have a full field, even during your opponent's turn, making guarding really easy. The critical scares your opponent into guarding if you can pull the megablast off midgame.
Agravain can be mixed with other clans to make megablasting faster. Tsukiyomi/Galahad lines are an easy 6 soul if you finish the g0-g3 chain. Mr. Invincible makes a good backup vanguard since he unflips any damage you might have to use. Just don't forget that only Gold Paladins count in his skill.
Other than his own Soul charging skill there are a few ways Gold Paladins can fill the soul:
Use the skill of Crimson Lion Cub, Kyrph. Even if you don't superior ride you still send 2 cards to the soul, and if you manage to superior ride while Grade 2 you can normal ride Agravain next turn without missing any of his soul charging
If built in a deck that can unflip damage with no problem (like by the effect of Mage of Calamity, Tripp used 5 times, or obtaining a Heal Trigger) and building 8 soul again, it becomes possible to stack his effect, to +2000 per Gold Paladin Rear-Guard, and 3 Criticals total.
If planning on using the skill of War-horse, Raging Storm frequently, a good strategy would be to play 6-8 draw triggers, you wont need that much guard when you become a 15k vg
Be aware that it is perfectly useless against a deck based on Stil Vampir, since that unit's skill will erase all the effects of Agravain's Megablast. Also, watch out for Dragonic Lawkeeper, as he can easily clear your field and disarm Agravain of that +5000 guarding power with just one CB.