The guards at the west gate will come out to fight, as well, if you fall back a bit.
A little fire at the north gate probably would have helped with the spread of poison... though that involve safety concerns, as well.
The magic ballista on the walls can be used in combat, though the soldiers won't necessarily stay out of the area of effect for where you are planning to attack.
A point in geomancer gives access to Midnight Oil and Summon Spider (which don't require intelligence) and Boulder Bash (ranged attack that doesn't need a line of sight).
There is an oil barrel by the harbour warehouse that you may be able to pre-place somewhere strategic with a bit of sneaking. There is one on the path before Cyseal, as well, right about where the characters generally are when entering combat; if it wasn't destroyed in that fight, you can go back and get it.
Like with the lower difficulty levels, the difficulty in tactician is higher at the start (as SniperHF said, a level difference is more significant then) and generally falls of as the game progresses. If you can get through the next couple of levels you should be fine.
