Need help with spellmaking.

I have a strong mage build at level 29 but want to rp as a warlock/necromancer. My plan is to shield up/summon at the start of a fight and to lay "curses" (damage over time spells) on enemies and let them get tied up in combat with my summon. Mana is not really a limiting factor. How should I go about crafting my spells? Lots of time with little damage and weakness? Lots of "smaller" spells piled onto each target? I also want to know if my spells can be applied to multiple targets at the same time. What do you think?