All you need is to unlock all 3 spells and then you can try hitting enemies by spamming skills this way : W -> E -> Q and if you have ult you can kill them with R too ! Also try to farm and use W and R to minions dont use other skills since they are not AoE . I use clarity when i have ult so that it doesnt eat my mana and cancel itself . Once you have Rylais you can use all your skills to escape for ganks and op players ( For example a fed Yi or Xin )

In the new patch Swain gets lower cooldown on his ult and increased basic attack range and Decrepify also gets range buffs . I think Swain was already a mage you wouldnt pick but i 've been getting legendary titles in everygame i play and this buff will make him one of the best mages for me !