There are several ways to use web.

1) CS based web, where you rely upon CS to overpower enemy TD. More MnS ranks = better web CS.

2) Bolt version of web. Uses Bolt AS vs critter DS ... requires 20 ranks of summoning lore. However, to greatly improve the odds of ensnaring the target, you also
need to train in spell aiming, the more the better. Note that spell aiming will also help other attack spell, such as spirit fire, as well as letting you use wizard wands.

3) You can just cast the spell and cover the floor of the room with webs. Any creature moving or attacking in the room has a chance to get stuck. Note that this version
is not player friendly and will ensnare non-grouped characters.