The possession bonus and condition bonus are easier to boost. It is the attacking bonus that is more difficult to boost unless you have more than the required number of attackers for a squad of 18-22 players; that is more than 3-4 strikers and 3-4 attacking midfielders. Having multi-position players will make boosting the bonuses easier in the case where those players have double or triple roles that are from back to front rather than side to side of the pitch; training a lot of such players together in 1 session will boost more than 1-2 bonus areas. But, take into consideration that to have such multi-position players, you usually have to spend a lot of green packs (50-120 per player) or spend a lot of tokens or a combination of those two.