Attikus weirdly enough plays a backfield role until about level 3 to 4. Make sure you aren’t picking him in a team comp with too many squishy characters. Ideally you’ll want another melee attacker who does well sustaining early (ie not El Dragon) to fill your front line position until you hit that mark. Until then, you are a shard gatherer/thrall spawner and, once you’ve gotten comfortable with his early weaknesses, sideline ambusher to discourage flanks or buildables being lost.
Level 3 brings you Tenacity, which absolutely explodes your damage output. Level 4 is Quickening Arc, eliminating Attikus’ movement speed weakness. Level 5 brings your choice of Hedronic Regeneration or his stack saving mutation (I prefer regen) and your Ult, and level 7 explodes your damage potential yet again with Swift Strikes.
Attack speed gear is also extremely important for Attikus, so in your early game stage, try to conserve some of them to get one of those activated. A zero cost shard generating gear will help with this, as you will take a long time to reach level 4 if you aren’t occasionally popping out an elite minion or turret. Attack damage gear is alright as well but Attack Speed takes priority as when you have Tenacity, faster attacks will naturally lead into yet more damage.
Once you have hit this point and hopefully activated some gear as well, Attikus becomes one of the most liberating characters to play. He has the damage potential of a Phoebe or El Dragon with the tankiness and body blocking ability of Montana. The right helix choices in his pounce and arc will ensure that your initial target has no hope of escape, and if you find yourself staring down a good chunk of the enemy team at once, your ultimate will save you from being CC’d and focused to death.