Use cards like Darkness Magician, Robert to set up the card you are going to put into your soul guage. Remember, Robert's ability activates at the beginning of the Main phase, with allows you to decided, not ONLY Trigger units, but for cards like this card, cards that you need to send to the soul to complete your combo.
Not a good card to spam at 4 because his effect is at its best situationally. This could also send a trigger unit that you don't want inside the soul to soul accidently. Also remember, you do not HAVE to use it's ability, so if all you need is a strong Booster/Attacker and you know the next card is a trigger, play this card without using its ability.