I like having the dragon swoop in for a breath weapon strafing attack (with surprise?), drop the devils to melee the PCs, and hang out somewhere that the victorious (or so they presume) devils can teleport back to him. For flair, he can circle in the air nearby. Or take potshots at the town's defenses; no reason to let the Guards reinforce the PCs, and grown-up dragons might well be overconfident.I'm having trouble with the Cult Strikes Back setup.
Need a location that might draw out the PCs? How about a temple of Bahamut that has advertised that they will train new Paladins to fight against Tiamat? Have the Cult send a (youngling) dracolich against that. It draws the PCs' attention, deliberately disrespects Bahamut, and warns the general populace against interfering with their betters' plans. The Council asks the PCs to be seen nearby in order to demonstrate 'action is being taken' to assure the recruits' safety.
This IS supposed to be an assassination, after all; prepare with the thought that some but not all of the PCs do die. Have an NPC cleric handy who can 'call 911' for a Resurrection spell. The text briefly mentions the advantages of being thought dead when you really are not. I've tried twice to really kill my PC group. Arauthator was prevented only because the Bard max'ed out a Staff of Healing. The first Cult assassin team was ganged up on by late-arriving PCs, but I had scattered the group and almost killed the Wizard - forcing him to flee and turn Invisible - before help arrived.
If the Cult is actively tracking the PCs around, they could be ambushed while in a Marketplace or somewhere else with lots of civilians and valuable property and things that might distract the PCs, or at least provide expendable cover for the devils. Some of the assassin squad would 'waste' a turn murdering civilians and drawing the PCs' attention - presumably so another devil can Sneak Attack them.
Would the Cult's version of James Bond reveal himself when/if the assassination fails to go as planned?