Wielding Seth's spear, Ava swung and stabbed at the Hollows with a lot of energy, and very little skill. At one point she was even swinging it like a baseball bat. If it wasn't for her 'holy light' that had extended over and enveloped the spear, causing the Hollows to explode into screeching dust on contact, then she wouldn't have been effective in the slightest in combat.
That's why Seth convinced her to reduce it. Not to remove it entirely, just focus on reducing it down to a small point. He wasn't going to entirely doom the fight for training, but he was going to use every opportunity he had.
His training mainly involved two things, having Ava focus her light on the very point of the spear, so that she couldn't baseball swing anymore, and then guiding her on how to hold and swing the spear properly.
